This seems related to this issue: Get(CurrentDate) function within FMP 10 Send Mail script step resolves as US formated date only
If monthname is the only language specific function that you need to be "selectable", you might consider writing your own function for it as it wouldn't be too complex to set up.
Case ( gLanguage = "English" ; Choose ( Month (datefield) ; "" ; "January" ; "February" ; "March" ; //and so on to "December") ;
gLanguage = "Francais" ; Choose ( Month (datefield) ; "" ; "Janvier" ; // and so on ) )
Another option suggested in the thread is to use separate files for the language specific portions of your solution. As noted in the other thread, that approach may or may not work for you.
In addition to what was stated before, the file stores the locale of the language that it was created/first used in : brand new or clone.
There's an option to use either the system format or file format within the File Options under the text category.
Calculations such as dayname and monthname will give a clue in which language the file was created and first used in.