And this reason was not also applied to other mentioned translation, some of which has even more letters in their names. For example, "Indonesian" as a word is longer than "Romanian", and "Russian" only 1 character shorter than "Romanian", but those names wasn't shortened.
I believe this is because some other words are much more difficult to shorten (more precisely, in their shortened form they may not convey the underlying meaning).
You have a point on this matter, now I also see that Portuguese translation was also shortened to "Portug". However, I find it not so common as, for example, "rus" which is shortened for Russian and is familiar almost to everyone.