Because XenForo works a lot with fixed widths your space for translations is limited to you without generating line breaks. Many languages, including german, can't be as much shortend as english without losing accuracy. If I were to shorten all german phrases to fit in a fixed width, the quality will always be worse than the english one. So non-english users will always have an inferior product than english users. And that can't be right. A language shouldn't adapt itself to a forum software, but a forum software should adapt itself to a language.
If I were to include CSS patches in language files I can increase the widths to allow more characters for a better quality translation. So it is possible to include CSS patches to language files that are applied when you install a language?
The CSS would go into a style. You would have to include the style with the language. The user will need to install both. Or you could include instructions for the user to manually add certain CSS to their existing style.