I apologize for necroposting yet again, but I'd like to add my two yen to this discussion.
I'm planning on localizing XenForo for a Japanese website. Ultimately I'll probably want to complete the localization job so that non-English-speaking Japanese users can enjoy using XenForo. But I personally don't really need a Japanese admin interface. In fact, I'd rather admin my site in English. So for the time being, I'm guessing my best option would be to use an incomplete language pack, which I don't consider an ideal way of doing things.
While testing out WordPress I got in the habit of using the
WP Native Dashboard plugin. This nice little addon allows each user to specify the language for the dashboard, so one can browse a blog in Japanese but admin it in English, or vice versa, etc. It's a great idea.
It's not perfect, though. WP Native Dashboard works well with a standard WP installation, but it isn't able to affect the behavior of other plugins. If I set my site to display in Japanese and my admin language to English, the control panel for my caching plugin still shows up in Japanese. So while I applaud the efforts of the plugin's author, I feel this is something that really needs to be built into WP itself to work right.
I think XenForo could benefit from the same sort of thinking. I'd like to see a setting that would allow each admin user to specify his or her preferred language for the admin UI. This would allow people like me to use my preferred language to admin without using an incomplete localization.
I don't know if such a setting would require any actual separation of resources, other than for the convenience of translators. And it may be that a plugin (like the one suggested by ajnos in the post above) is all that's needed. If a plugin will result in inconsistent behavior of the sort I experienced with WP Native Dashboard, however, I'd prefer that this functionality be built into XenForo.