Hi, what do you mean by common? Geographically, English is common. But if it is counted by population, Manderin is common. I teach very good Manderin. Contact me.

hi Both "onlar güclüdür" and "onlar güclüdürlər" are correct in azeri and are used, but "onlar güclüdür" is the best one to use both in formal and informal speech. Güc (power, strength) is spelled as g-ü-c in azeri, in turkish it is spelled as g-ü-ç. The third version - "güclüler" is correct in turkish, but not with "ə" but "e" in -"lər".