Suggestion: Separate Interface and Dialogue Language

[Rongway]

On the other hand, the main database for the game (The Lodestone) is readily available in german, as are most guides.
I play the game in german, but know most skill's/dungeon's/etc. english names, as they are similar (or almost identical), e.g. the lancer's skill 'Vorpal Thrust' would be 'Vorpalstoß' in german.

Yes, what I do when I want to see what actions are called in other languages is I go to the page in the job guide in my language (english) and then change the subdomain in the URL to the language I want. This also works on the Lodestone for quest and item entries.

However, I can understand OP's desire to play with mixed language settings, as there are also some speakers of other languages for whom the game is only available in second languages for them, and the dialog may be easier for them to follow in French or German but most guides for the game are written in English.

I don't think it would be particularly difficult to split the localization dictionaries into two sections so that the dialog and the interface can be in two different lanugages; however, a thorough job would be time consuming and probably not viewed as a profitable endeavor.