is there any "official" limit on the length/number of words of a non-translatable proper name?
If, say, a company' name is composed of several words (name, legal form abbreviation, registered office, etc.) CF accepts it on the general list of non-translatable resources but fails to mark it in the source segment and place it on the F4 list.
Would it be possible to fix it?
There is no such limit. However, in certain cases (e.g. when your non-translatable spans some tags), CT cannot recognize them as the program searches for non-translatables between the tags.