That code is managed separately and has absolutely nothing with languages. You can try any other page (e.g. search or item/ad) if you are bothered with. Does it work?
Also, Osclass 3.7.4 should not be used, as many bugs were fixed in 3.8.0, but as far as I can remember, it also works fine for languages.
Anyway, I have installed 3.7.4 and languages work properly with cleared cookies (what history has to do with it??). Have you tried this on a different server, hosting, localhost, different browser and computer?