[dtdesign]

The ACP language import used the same code path as package installations. It only updated phrases owned by the selected package and created unknown phrases under it, so a full translation had to be uploaded once per package.

The import now only provides values for existing phrases, and the package selection is gone:

• A phrase is known if a package installed it into de or en. Its package and category are taken from there. Phrases missing from the target language are inserted.
• Unknown phrases and categories are ignored silently, and so is the <delete> section.
• Phrases not installed by a package, such as manually added ones, are left untouched.
• Changing a phrase that has a custom value flags it for review, the same as a package update does.

Closes #6516