I am not sure that being able to turn off folding is a good idea, IIUC parts of symbols handling uses it to locate the current block/function/construct. There may be other places its used as its the only part of Geany that provides structure information.
Havn't looked at the changes but IRO your questions:
Should I make a different PR for this? (asusming it even is desired!)
Sounds like its sensible to be part of the change.
Also, should I or would you suggest that I also modify the manual to document the additions?
Of course.
Should I add something about the new UI options in the language/translation files?
IIUC should happen automatically if the strings are marked for translation.