Configurable labels for language selector

[wolfgangmm]

The label shown by the language selector in the menubar should be configurable and not limited in width.

Support an explicit, arbitrary label to be defined in languages.json, e.g.:

{
    "zh_Hant": {
        "label": "繁體中文",
        "abbreviation": "zh"
    },
    "en": {
        "label": "English",
        "abbreviation": "en"
    }
}

"language-label": "abbreviation" in config.json will use abbreviation for display. Backwards compatibility with old languages.json is preserved.