This moves language settings to a new settings panel, and adds a checkbox covered with warnings:

Enabling it activates translation mode. Translated strings appear with an underline: violet for untranslated, blue for file-translated, and green for user-translated:

If you shift-click any translatable text, you can provide a translation:

None of this works yet or saves anywhere, but it seems to behave properly, and essentially solve the hardest UI part of doing contextual translation.