We can update older titles with their original title, I do it from time to time when I encounter older titles. I'm not sure it is very useful to update entries just to add the flag, but as it seems that more people are for it rather than against it I suppose we can add it as well.

Transliterated title is the original one written in latin characters, so I'm not sure what you mean for German/Italian/French, etc. I suppose you rather mean International title then?
If so, in some cases it can be used for these titles also if the movie is more known abroad with that title. If IMDb mentions an English title as "Worldwide", or if the English title is used in most of the countries.

A good example of German title that benefits having an International title added: https://www.imcdb.org/movie_88323-Die-unendliche-Geschichte.html
But I'm not sure that putting the German flag for that one is really useful.