Adds missing and sorts existing language items

Also:

* Improves some language items.
* Moves `wcf.acp.user.rank` language items into `wcf.acp.user` since forth-level language categories aren't supported.
* Adds missing (i.e. currently untranslated) English language items, but they are commented out, thus not installed and still "appear to be missing". If you want to translate them, you can simply search for `<!-- todo: ` in `en.xml`.