diff --git a/Package.swift b/Package.swift
index 4d907e4..afd910f 100644
--- a/Package.swift
+++ b/Package.swift
@@ -16,5 +16,6 @@ let package = Package(
       .target(name: "W3WSwiftCore", dependencies: []),
       .testTarget(name: "w3w-swift-typesTests", dependencies: ["W3WSwiftCore"]),
       .testTarget(name: "w3w-swift-Tests", dependencies: ["W3WSwiftCore"]),
+      .testTarget(name: "languages-Tests", dependencies: ["W3WSwiftCore"])
     ]
 )
diff --git a/Sources/W3WSwiftCore/Localization/W3WTranslationsProtocol.swift b/Sources/W3WSwiftCore/Localization/W3WTranslationsProtocol.swift
index 259726a..8e45cd1 100644
--- a/Sources/W3WSwiftCore/Localization/W3WTranslationsProtocol.swift
+++ b/Sources/W3WSwiftCore/Localization/W3WTranslationsProtocol.swift
@@ -6,7 +6,7 @@
 //
 
 
-public protocol W3WTranslationsProtocol {
+public protocol W3WTranslationsProtocol: W3WAvailableLanguageProtocol {
   
   /// given a translation id return the translation for the given device locale
   /// - Parameters:
@@ -21,6 +21,15 @@ public protocol W3WTranslationsProtocol {
   
 }
 
+public protocol W3WLanguageSelectionProtocol  {
+  /// to set RFCLanguage
+  func set(language: W3WRfcLanguage)
+}
+
+/// list out all available RFCLanguages
+public protocol W3WAvailableLanguageProtocol {
+  func availableLanguages() -> [W3WRfcLanguage]
+}
 
 public extension W3WTranslationsProtocol {
   
@@ -42,3 +51,10 @@ public extension W3WTranslationsProtocol {
     return String(format: localized, arguments)
   }
 }
+
+public extension W3WAvailableLanguageProtocol {
+  /// default convenience func for available languages, should override when in need
+  func availableLanguages() -> [W3WRfcLanguage] {
+    return []
+  }
+}
diff --git a/Sources/W3WSwiftCore/RfcLanguage/W3WRfcLanguage+W3WLanguage.swift b/Sources/W3WSwiftCore/RfcLanguage/W3WRfcLanguage+W3WLanguage.swift
new file mode 100644
index 0000000..7b379e8
--- /dev/null
+++ b/Sources/W3WSwiftCore/RfcLanguage/W3WRfcLanguage+W3WLanguage.swift
@@ -0,0 +1,29 @@
+//
+//  File.swift
+//  w3w-swift-core
+//
+//  Created by Kaley Nguyen on 16/9/25.
+//
+
+import Foundation
+
+public extension W3WRfcLanguageProtocol {
+  /// convert W3WRfcLanguage to W3WLanguage
+  func toW3wLanguage() -> W3WLanguage? {
+    guard let code, !code.isEmpty else {
+      return nil
+    }
+    return W3WBaseLanguage(locale: self.shortIdentifier)
+  }
+}
+
+@available(iOS 13.0.0, *)
+@available(watchOS 6.0.0, *)
+extension W3WBaseLanguage: W3WRfcLanguageConvertable {
+  /// convert W3WBaseLanguage to any W3WRfcLanguageProtocol
+  public func toRfcLanguage() -> some W3WRfcLanguageProtocol {
+    let parsedLocale = Locale(identifier: locale)
+    return W3WRfcLanguage(locale: parsedLocale)
+  }
+}
+
diff --git a/Sources/W3WSwiftCore/RfcLanguage/W3WRfcLanguage+W3WSdkLanguage.swift b/Sources/W3WSwiftCore/RfcLanguage/W3WRfcLanguage+W3WSdkLanguage.swift
new file mode 100644
index 0000000..f1937ac
--- /dev/null
+++ b/Sources/W3WSwiftCore/RfcLanguage/W3WRfcLanguage+W3WSdkLanguage.swift
@@ -0,0 +1,31 @@
+//
+//  File.swift
+//  w3w-swift-core
+//
+//  Created by Kaley Nguyen on 21/11/25.
+//
+
+import Foundation
+#if canImport(w3w)
+import w3w
+
+public extension W3WRfcLanguageProtocol {
+  /// convert W3WRfcLanguage to W3WSdkLanguage
+  func toW3wSdkLanguage() throws -> W3WSdkLanguage? {
+    guard let code, !code.isEmpty else {
+      return nil
+    }
+    return try W3WSdkLanguage(shortIdentifier)
+  }
+}
+
+@available(iOS 13.0.0, *)
+@available(watchOS 6.0.0, *)
+extension W3WSdkLanguage: W3WRfcLanguageConvertable {
+  /// convert W3WSdkLanguage to any W3WRfcLanguageProtocol
+  public func toRfcLanguage() -> some W3WRfcLanguageProtocol {
+    return W3WRfcLanguage(locale: Locale(identifier: locale))
+  }
+}
+#endif
+
diff --git a/Sources/W3WSwiftCore/RfcLanguage/W3WRfcLanguage.md b/Sources/W3WSwiftCore/RfcLanguage/W3WRfcLanguage.md
new file mode 100644
index 0000000..da2e337
--- /dev/null
+++ b/Sources/W3WSwiftCore/RfcLanguage/W3WRfcLanguage.md
@@ -0,0 +1,29 @@
+# W3WRfcLanguage
+
+A lightweight representation of an RFC 5646 language tag used in w3w-swift-core. It models:
+
+- language code (ISO 639; prefer 2-letter when available)
+- optional script code (4 letters, Titlecase)
+- optional region code (2 letters or 3 digits)
+
+It provides:
+- A protocol `W3WRfcLanguageProtocol` that any type can conform to
+- A concrete struct `W3WRfcLanguage`
+- An extension that makes `Locale.Language` conform to the protocol on iOS 16+/watchOS 9+
+- Utilities to build identifiers and parse from strings
+
+## Availability
+
+- `Locale.Language` conformance requires iOS 16+ or watchOS 9+.
+- `W3WRfcLanguage` itself is available on all supported platforms; when running on iOS < 16/watchOS < 9, script extraction from `Locale` may be unavailable and remain `nil`.
+
+## Types
+
+### W3WRfcLanguageProtocol
+```swift
+public protocol W3WRfcLanguageProtocol: Equatable {
+  var code: String? { get }        // ISO 639 language code
+  var scriptCode: String? { get }  // 4-letter Titlecase script code (e.g. "Latn", "Hans")
+  var regionCode: String? { get }  // 2-letter or 3-digit region (e.g. "US", "419")
+}
+
diff --git a/Sources/W3WSwiftCore/RfcLanguage/W3WRfcLanguage.swift b/Sources/W3WSwiftCore/RfcLanguage/W3WRfcLanguage.swift
new file mode 100644
index 0000000..2a14558
--- /dev/null
+++ b/Sources/W3WSwiftCore/RfcLanguage/W3WRfcLanguage.swift
@@ -0,0 +1,150 @@
+//
+//  File.swift
+//  w3w-swift-core
+//
+//  Created by Kaley Nguyen on 15/8/25.
+//
+
+import Foundation
+
+/// language[-script][-region]
+/// - language = ISO 639
+///   Prefer ISO 639-1 (2-letter) whenever it exists
+///   Only fall back to ISO 639-2 (3-letter) if a language has no 2-letter code
+/// - script (optional, 4 letters, titlecased)
+/// - region (optional, 2 letters or 3-digit number)
+public struct W3WRfcLanguage: W3WRfcLanguageProtocol {
+  /// default language is "en". name's default can be changed externally to use a different language by an app if nessesary
+  public static var `default` = W3WRfcLanguage(code: "en")
+  
+  public var code: String?
+  public var scriptCode: String?
+  public var regionCode: String?
+  
+  public init(locale: Locale) {
+    if #available(iOS 16, *), #available(watchOS 9, *) {
+      // from iOS 16 supports getting script
+      self.init(from: locale.language)
+    } else {
+      // no script for iOS < 16
+      self.init(code: locale.languageCode,
+                scriptCode: locale.scriptCode,
+                regionCode: locale.regionCode)
+    }
+  }
+  
+  public init(
+    code: String? = nil,
+    scriptCode: String? = nil,
+    regionCode: String? = nil
+  ) {
+    self.code = code
+    self.scriptCode = scriptCode
+    self.regionCode = regionCode
+  }
+  
+  @available(iOS 16, *)
+  @available(watchOS 9, *)
+  public init(from language: Locale.Language) {
+    self.code = language.code
+    self.scriptCode = language.scriptCode
+    self.regionCode = language.regionCode
+  }
+}
+
+public extension W3WRfcLanguage {
+  /// Initialize from a string
+  init(from string: String) {
+    // Normalize separators
+    let normalized = string
+      .trimmingCharacters(in: .whitespacesAndNewlines)
+      .replacingOccurrences(of: "_", with: "-")
+    
+    // Handle “Base” or empty strings, check if string is a valid locale string
+    guard !normalized.isEmpty, string.isValidLocale else {
+      self.init(code: nil, scriptCode: nil, regionCode: nil)
+      return
+    }
+    
+    if #available(iOS 16, *), #available(watchOS 9, *) {
+      self.init(from: Locale.Language(identifier: normalized))
+    } else {
+      // Split into parts (e.g. zh-Hans-CN → ["zh", "Hans", "CN"])
+      let parts = normalized.split(separator: "-").map(String.init)
+      
+      var langCode: String?
+      var script: String?
+      var region: String?
+      
+      if parts.count > 0 {
+        langCode = parts[0]
+      }
+      
+      if parts.count > 1 {
+        let part = parts[1]
+        // Detect if this is a script code (Titlecase and 4 letters)
+        if part.count == 4, part.first?.isUppercase == true {
+          script = part
+          if parts.count > 2 {
+            region = parts[2]
+          }
+        } else {
+          region = part
+        }
+      }
+      
+      self.init(code: langCode, scriptCode: script, regionCode: region)
+    }
+  }
+}
+
+public extension W3WRfcLanguageProtocol {
+  /// full version: code - script - region
+  var identifier: String {
+    return [code, scriptCode, regionCode]
+      .compactMap { $0 }
+      .joined(separator: "-")
+  }
+  /// short : code - region
+  var shortIdentifier: String {
+    return [code, regionCode]
+      .compactMap { $0 }
+      .joined(separator: "-")
+  }
+}
+
+@available(watchOS 9, *)
+@available(iOS 16, *)
+extension Locale.Language : W3WRfcLanguageProtocol {
+  public var code: String? {
+    return languageCode?.identifier
+  }
+  
+  public var scriptCode: String? {
+    return self.script?.identifier
+  }
+  
+  public var regionCode: String? {
+    return region?.identifier
+  }
+}
+
+extension W3WRfcLanguage {
+  public var nativeName: String? {
+    return LanguageUtils.getLanguageName(forLocale: identifier, inLocale: identifier)
+  }
+  
+  public var name: String? {
+    return LanguageUtils.getLanguageName(forLocale: identifier, inLocale: Self.default.identifier)
+  }
+  
+  /// get name of the language in any particular locale
+  func name(in locale: String) -> String? {
+    return LanguageUtils.getLanguageName(forLocale: identifier, inLocale: locale)
+  }
+  
+  /// get name of the language in any other language
+  func name(in language: W3WRfcLanguage) -> String? {
+    return name(in: language.identifier)
+  }
+}
diff --git a/Sources/W3WSwiftCore/RfcLanguage/W3WRfcLanguageProtocol.swift b/Sources/W3WSwiftCore/RfcLanguage/W3WRfcLanguageProtocol.swift
new file mode 100644
index 0000000..41ea5bb
--- /dev/null
+++ b/Sources/W3WSwiftCore/RfcLanguage/W3WRfcLanguageProtocol.swift
@@ -0,0 +1,20 @@
+//
+//  File.swift
+//  w3w-swift-core
+//
+//  Created by Kaley Nguyen on 21/11/25.
+//
+
+import Foundation
+
+public protocol W3WRfcLanguageProtocol: Equatable {
+  var code: String? { get }
+  var scriptCode: String? { get }
+  var regionCode: String? { get }
+}
+
+/// protocol to convert any language to W3WRfcLanguage
+public protocol W3WRfcLanguageConvertable<Language> {
+  associatedtype Language: W3WRfcLanguageProtocol
+  func toRfcLanguage() -> Language
+}
diff --git a/Sources/W3WSwiftCore/Types/Util/LanguageUtils.swift b/Sources/W3WSwiftCore/Types/Util/LanguageUtils.swift
new file mode 100644
index 0000000..cd6be1d
--- /dev/null
+++ b/Sources/W3WSwiftCore/Types/Util/LanguageUtils.swift
@@ -0,0 +1,35 @@
+//
+//  LanguageUtils.swift
+//  w3w-swift-core
+//
+//  Created by Kaley Nguyen on 14/11/25.
+//
+import Foundation
+
+public class LanguageUtils {
+  /// gets the langauge name for a locale in the language specified by 'inLocale'
+  /// Parameters
+  ///   - forLocale: locale to find the language of
+  ///   - inLocale: locale to translate the langauge name into
+  public static func getLanguageName(forLocale: String, inLocale: String) -> String? {
+    let inLocaleObj  = NSLocale(localeIdentifier: inLocale)
+    let forLocaleObj = NSLocale(localeIdentifier: forLocale)
+    
+    var languageName = inLocaleObj.localizedString(forLanguageCode: forLocale) ?? ""
+    
+    if forLocale.count > 2 {
+      if let countryCode = forLocaleObj.countryCode {
+        languageName += " (" + (inLocaleObj.localizedString(forCountryCode: countryCode) ?? "") + ")"
+      }
+    }
+    
+    return languageName
+  }
+}
+
+extension String {
+  var isValidLocale: Bool {
+    let normalized = self.replacingOccurrences(of: "-", with: "_")
+    return Locale.availableIdentifiers.contains(normalized)
+  }
+}
diff --git a/Sources/W3WSwiftCore/Types/Values/W3WLanguage.swift b/Sources/W3WSwiftCore/Types/Values/W3WLanguage.swift
index acbf9e5..cbd2b4d 100644
--- a/Sources/W3WSwiftCore/Types/Values/W3WLanguage.swift
+++ b/Sources/W3WSwiftCore/Types/Values/W3WLanguage.swift
@@ -32,12 +32,19 @@ public protocol W3WLanguage {
 
 
 extension W3WLanguage {
+  /// To get the first 2 (or 3 max) in a locale string separated by "-", i.e: en-US / en
+  static func extractCode(from string: String) -> String {
+    let parts = string.split(separator: "-").map(String.init)
+    if !parts.isEmpty {
+      return parts[0]
+    }
+    return ""
+  }
   
   public static func getLanguageCode(from: String) -> String {
-    return String(from.prefix(2))
+    return extractCode(from: from)
   }
   
-  
   public static func getLanguageRegion(from: String) -> String {
     if from.count == 5 {
       if String(Array(from)[2]) == "_" {
@@ -51,35 +58,14 @@ extension W3WLanguage {
     return ""
   }
 
-  
-  /// gets the langauge name for a locale in the language specified by 'inLocale'
-  /// Parameters
-  ///   - forLocale: locale to find the language of
-  ///   - inLocale: locale to translate the langauge name into
-  public static func getLanguageName(forLocale: String, inLocale: String) -> String? {
-    let inLocaleObj  = NSLocale(localeIdentifier: inLocale)
-    let forLocaleObj = NSLocale(localeIdentifier: forLocale)
-    
-    var langaugeName = inLocaleObj.localizedString(forLanguageCode: forLocale) ?? ""
-    
-    if forLocale.count > 2 {
-      if let countryCode = forLocaleObj.countryCode {
-        langaugeName += " (" + (inLocaleObj.localizedString(forCountryCode: countryCode) ?? "") + ")"
-      }
-    }
-    
-    return langaugeName
-  }
-
-  
   public func getDeviceLanguages() -> [W3WLanguage] {
     var langauges = [W3WLanguage]()
     
     for locale_code in NSLocale.availableLocaleIdentifiers.sorted() {
       let language = W3WBaseLanguage(
         locale: locale_code,
-        name: Self.getLanguageName(forLocale: locale_code, inLocale: self.locale) ?? "",
-        nativeName: Self.getLanguageName(forLocale: locale_code, inLocale: locale_code) ?? ""
+        name: LanguageUtils.getLanguageName(forLocale: locale_code, inLocale: self.locale) ?? "",
+        nativeName: LanguageUtils.getLanguageName(forLocale: locale_code, inLocale: locale_code) ?? ""
       )
       langauges.append(language)
     }
@@ -133,19 +119,19 @@ public struct W3WBaseLanguage: W3WLanguage, ExpressibleByStringLiteral {
   ///   - nativeName: Name of the langiage in that language
   public init(code: String, name: String = "", nativeName: String = "") {
     if name == "" {
-      self.name = Self.getLanguageName(forLocale: code, inLocale: "en")
+      self.name = LanguageUtils.getLanguageName(forLocale: code, inLocale: "en")
     } else {
       self.name = name
     }
     
     if nativeName == "" {
-      self.nativeName = Self.getLanguageName(forLocale: code, inLocale: code)
+      self.nativeName = LanguageUtils.getLanguageName(forLocale: code, inLocale: code)
     } else {
       self.nativeName = nativeName
     }
     
-    self.code       = String(code.prefix(2))
-    self.locale     = String(code.prefix(2))
+    self.code       = Self.extractCode(from: code)
+    self.locale     = Self.extractCode(from: code)
   }
   
   
@@ -156,26 +142,41 @@ public struct W3WBaseLanguage: W3WLanguage, ExpressibleByStringLiteral {
   ///   - nativeName: Name of the langiage in that language
   public init(locale: String, name: String = "", nativeName: String = "") {
     if name == "" {
-      self.name = Self.getLanguageName(forLocale: locale, inLocale: "en")
+      self.name = LanguageUtils.getLanguageName(forLocale: locale, inLocale: "en")
     } else {
       self.name = name
     }
     
     if nativeName == "" {
-      self.nativeName = Self.getLanguageName(forLocale: locale, inLocale: locale)
+      self.nativeName = LanguageUtils.getLanguageName(forLocale: locale, inLocale: locale)
     } else {
       self.nativeName = nativeName
     }
     
-    self.code       = String(locale.prefix(2))
+    self.code       = Self.extractCode(from: locale)
     self.locale     = locale
   }
   
+  public init(code: String, locale: String, name: String = "", nativeName: String = "") {
+    self.code = Self.extractCode(from: code)
+    self.locale = Self.extractCode(from: locale)
+    if name == "" {
+      self.name = LanguageUtils.getLanguageName(forLocale: locale, inLocale: "en")
+    } else {
+      self.name = name
+    }
+    
+    if nativeName == "" {
+      self.nativeName = LanguageUtils.getLanguageName(forLocale: locale, inLocale: locale)
+    } else {
+      self.nativeName = nativeName
+    }
+  }
+  
   
   public init(stringLiteral: String) {
     self.init(locale: stringLiteral)
   }
   
-
   static public let english = W3WBaseLanguage(locale: "en_gb")
 }
diff --git a/Tests/languages-Tests/W3wRfcLanguage_w3wLanguage_tests.swift b/Tests/languages-Tests/W3wRfcLanguage_w3wLanguage_tests.swift
new file mode 100644
index 0000000..62c4c6d
--- /dev/null
+++ b/Tests/languages-Tests/W3wRfcLanguage_w3wLanguage_tests.swift
@@ -0,0 +1,145 @@
+//
+//  W3wRfcLanguage_w3wLanguage_tests.swift
+//  w3w-swift-core
+//
+//  Created by Kaley Nguyen on 21/11/25.
+//
+
+import Testing
+@testable import W3WSwiftCore
+import w3w
+
+@Suite
+struct W3wRfcLanguage_W3wLanguage_Tests {
+  
+  @Test(arguments: ["fr", "en", "ja", "ko", "pt", "de"])
+  func convertLanguageToRfcLanguage_LanguageOnly_Success(lang: String) {
+    let testLang = W3WBaseLanguage(locale: lang)
+    let rfcLang = testLang.toRfcLanguage()
+    
+    #expect(rfcLang.regionCode == nil)
+    #expect(rfcLang.code == testLang.code)
+    
+    let rfcLang2 = W3WRfcLanguage(from: lang)
+    #expect(rfcLang2.code == rfcLang.code)
+    #expect(rfcLang2.regionCode == rfcLang.regionCode)
+    #expect(rfcLang2.scriptCode == rfcLang.scriptCode)
+  }
+  
+  @Test(arguments: [
+    "en-US", "en-GB", "fr-CA", "pt-BR", "pt-PT",
+    "es-MX", "es-ES", "vi-VN", "ja-JP", "ko-KR"
+  ])
+  func convertLanguageToRfcLangage_LanguageRegion_Success(lang: String) {
+    let testLang = W3WBaseLanguage(locale: lang)
+    let rfcLang = testLang.toRfcLanguage()
+    
+    #expect(rfcLang.regionCode != nil)
+    #expect(rfcLang.code == testLang.code)
+    
+    let rfcLang2 = W3WRfcLanguage(from: lang)
+    #expect(rfcLang2.code == rfcLang.code)
+    #expect(rfcLang2.regionCode == rfcLang.regionCode)
+    #expect(rfcLang2.scriptCode == rfcLang.scriptCode)
+    
+  }
+  
+  @Test(arguments: [
+    "zh-Hans", "zh-Hant", "sr-Cyrl", "sr-Latn",
+    "uz-Cyrl", "uz-Latn"
+  ])
+  func convertLanguageToRfcLangage_LanguageScript_Success(lang: String) {
+    let testLang = W3WBaseLanguage(locale: lang)
+    let rfcLang = testLang.toRfcLanguage()
+    
+    #expect(rfcLang.regionCode == nil)
+    if #available(iOS 16, *) {
+      #expect(rfcLang.scriptCode != nil)
+    }
+    
+    let rfcLang2 = W3WRfcLanguage(from: lang)
+    #expect(rfcLang2.code == rfcLang.code)
+    #expect(rfcLang2.regionCode == rfcLang.regionCode)
+    if #available(iOS 16, *) {
+      #expect(rfcLang2.scriptCode == rfcLang.scriptCode)
+    }
+  }
+  
+  @Test(arguments: [
+    "zh-Hans-CN", "zh-Hant-TW", "sr-Cyrl-RS",
+    "sr-Latn-BA", "uz-Latn-UZ"
+  ])
+  func convertLanguageToRfcLangage_LanguageScriptRegion_Success(lang: String) {
+    let testLang = W3WBaseLanguage(locale: lang)
+    let rfcLang = testLang.toRfcLanguage()
+    
+    #expect(rfcLang.regionCode != nil)
+    if #available(iOS 16, *) {
+      #expect(rfcLang.scriptCode != nil)
+    }
+    #expect(rfcLang.code == testLang.code)
+    
+    let rfcLang2 = W3WRfcLanguage(from: lang)
+    #expect(rfcLang2.code == rfcLang.code)
+    #expect(rfcLang2.regionCode == rfcLang.regionCode)
+    if #available(iOS 16, *) {
+      #expect(rfcLang2.scriptCode == rfcLang.scriptCode)
+    }
+  }
+  
+  @Test(arguments: [
+    "vi", "af", "id", "ms", "de",
+    "en-AU", "en-GB", "en-US", "en-IN",
+    "es-ES", "es-MX", "sw", "nl",
+    "pt-BR", "pt-PT", "ro", "fi",
+    "sv", "xh", "zu", "bg", "mn",
+    "ru", "kk", "ko", "ja", "ar"
+  ])
+  func convertLanguageToRfcLangage_CurrentLanguageList_Success(lang: String) {
+    let rfcLang = W3WRfcLanguage(from: lang)
+    
+    #expect(rfcLang.code != nil)
+    if #available(iOS 16, *) {
+      #expect(rfcLang.scriptCode != nil)
+    }
+    if lang.count > 3 {
+      #expect(rfcLang.regionCode != nil)
+    }
+  }
+  
+  @Test(arguments: [
+    "en_US", "fr_FR", "pt_BR", "zh_CN", "zh_TW",
+    "sr_Cyrl_RS"
+  ])
+  func convertLanguageToRfcLangage_UnderscoreFormats_AppleLegacy_Success(lang: String) {
+    let rfcLang = W3WRfcLanguage(from: lang)
+    let w3wLang = rfcLang.toW3wLanguage()
+    
+    #expect(w3wLang?.code == rfcLang.code)
+  }
+  
+  @Test(arguments: [
+    "haw-US",
+    "yue-Hant",
+    "gsw-CH"
+  ])
+  func convertLanguageToRfcLanguage_3CharactersCode_Success(lang: String) {
+    let rfcLang = W3WRfcLanguage(from: lang)
+    #expect(rfcLang.code?.count == 3)
+  }
+  
+  @Test(arguments: [
+    "xx", "xxx", "123",
+    "en-ABCDE", "sr-OOO", "hr-12ab",
+    "en-USS", "fr-1234", "es-1X", "zh-Hans-CNN",
+    "CN-zh", "Cyrl-sr", "US-en",
+    "en-US-extra", "sr-Cyrl-RS-foo",
+    "-en", "en--US", "en-", "en- -US"
+  ])
+  func convertLanguageToRfcLanguage_InvalidLanguage_Failure(lang: String) {
+    let rfcLang = W3WRfcLanguage(from: lang)
+    #expect(rfcLang.code == nil)
+    #expect(rfcLang.scriptCode == nil)
+    #expect(rfcLang.regionCode == nil)
+  }
+}
diff --git a/Tests/languages-Tests/W3wRfcLanguage_w3wSdkLanguage_tests.swift b/Tests/languages-Tests/W3wRfcLanguage_w3wSdkLanguage_tests.swift
new file mode 100644
index 0000000..fcafa0b
--- /dev/null
+++ b/Tests/languages-Tests/W3wRfcLanguage_w3wSdkLanguage_tests.swift
@@ -0,0 +1,89 @@
+//
+//  w3wRfcLanguage_w3wSdkLanguage_tests.swift
+//  w3w-swift-core
+//
+//  Created by Kaley Nguyen on 21/11/25.
+//
+
+import Testing
+@testable import W3WSwiftCore
+import w3w
+
+@Suite
+struct W3wRfcLanguage_W3wSdkLanguage_Tests {
+  
+  @Test
+  func convertSdkLanguageToRfcLanguage_Success() throws {
+    let sdkLanguage = try W3WSdkLanguage("fr_fr")
+    let convertedRfcLanguage = try #require(sdkLanguage.toRfcLanguage() as? W3WRfcLanguage)
+    let rfcLanguage = W3WRfcLanguage(from: "fr_FR")
+    
+    #expect(convertedRfcLanguage == rfcLanguage)
+    #expect(convertedRfcLanguage.code == "fr")
+    #expect(convertedRfcLanguage.regionCode == "FR")
+    
+    if #available(iOS 16, *) {
+      #expect(convertedRfcLanguage.scriptCode != nil)
+    }
+  }
+  
+  @Test
+  func convertSdkLanguageToRfcLanguage_CodeOnly_Success() throws {
+    let sdkLanguage = try W3WSdkLanguage("ja")
+    let convertedRfcLanguage = try #require(sdkLanguage.toRfcLanguage() as? W3WRfcLanguage)
+    let rfcLanguage = W3WRfcLanguage(from: "ja")
+    
+    #expect(convertedRfcLanguage == rfcLanguage)
+    #expect(convertedRfcLanguage.code == "ja")
+    #expect(convertedRfcLanguage.regionCode == nil)
+    
+    if #available(iOS 16, *) {
+      #expect(convertedRfcLanguage.scriptCode != nil)
+    }
+  }
+  
+  @Test
+  func convertRfcLanguageToSdkLanguage_Success() throws {
+    let rfcLanguage = W3WRfcLanguage(from: "zh-Hans")
+    let sdkLanguage = try rfcLanguage.toW3wSdkLanguage()
+    
+    #expect(sdkLanguage != nil)
+    #expect(sdkLanguage?.code == "zh")
+  }
+  
+  @Test
+  func convertRfcLanguageToSdkLanguage_Code3Characters_Failure() {
+    let rfcLanguage = W3WRfcLanguage(from: "yue-Hans")
+    
+    #expect(throws: Error.self) {
+      _ = try rfcLanguage.toW3wSdkLanguage()
+    }
+  }
+  
+  @Test
+  func convertSdkLanguageToRfcLanguage_CodeScriptRegion_Failure() {
+    let rfcLanguage = W3WRfcLanguage(from: "zh-Hans-CN")
+    
+    #expect(throws: Error.self) {
+      _ = try rfcLanguage.toW3wSdkLanguage()
+    }
+  }
+  
+  /// Compare W3WBaseLanguage & W3WSdkLanguage by converting to W3WRfcLanguage
+  @Test
+  func matchingRfcLanguage() throws {
+    let string = "sv-SE"
+    let sdkString = "sv_se"
+    
+    let w3wBaseLanguage = W3WBaseLanguage(locale: string)
+    let w3wSdkLanguage = try W3WSdkLanguage(sdkString)
+    
+    let firstLang = try #require(w3wBaseLanguage.toRfcLanguage() as? W3WRfcLanguage)
+    let secondLang = try #require(w3wSdkLanguage.toRfcLanguage() as? W3WRfcLanguage)
+    
+    #expect(firstLang == secondLang)
+    #expect(firstLang.code == secondLang.code)
+    #expect(firstLang.regionCode == secondLang.regionCode)
+  }
+}
+
diff --git a/Tests/languages-Tests/w3wRfcLanguage_tests.swift b/Tests/languages-Tests/w3wRfcLanguage_tests.swift
new file mode 100644
index 0000000..940f2aa
--- /dev/null
+++ b/Tests/languages-Tests/w3wRfcLanguage_tests.swift
@@ -0,0 +1,29 @@
+//
+//  w3wRfcLanguage_tests.swift
+//  w3w-swift-core
+//
+//  Created by Kaley Nguyen on 3/12/25.
+//
+
+import Testing
+@testable import W3WSwiftCore
+import w3w
+
+@Suite
+struct w3wRfcLanguage_tests {
+  
+  @Test func initRfcLanguage() async throws {
+    let lang = W3WRfcLanguage(locale: Locale(identifier: "fr-FR"))
+    #expect(lang.code == "fr")
+    #expect(lang.nativeName == "français (France)")
+    #expect(lang.name == "French (France)")
+    #expect(lang.name(in: "ja") == "フランス語 (フランス)")
+  }
+  
+  @Test func initRfcLanguage_changeDefault() async throws {
+    let lang = W3WRfcLanguage(locale: Locale(identifier: "fr-FR"))
+    W3WRfcLanguage.default = W3WRfcLanguage(from: "de")
+    #expect(lang.code == "fr")
+    #expect(lang.name == "Französisch (Frankreich)")
+  }
+}