So there is no way of knowing, if any non-cracked language might just be gibberish.

I am really surprised that even languages like sumerian and ancient egyptian and ancient south american languages were translated. I guess language has to follow certain rules, and these allow them to be translated.

this in turn would mean that our own contemporary languages like english, han, hindi would be deciphered by alien invadors (I know not likely lol). In turn we would be able to decipher any alien language we might come across in the future and be it just in the form of an intercepted age old radio broadcast from a distant system.