Well, non-citizens (i.e. resident foreigners) have no inherent right to stay at a host country, even ones legally immigrated or born there. They are more or less there at the grace of the host country. Now, I dont advocate the sudden mass removal of over 2m people. This would be a humanitarian and besides also economical catastrophe. But I would advocate making removing them easier or making setting them under pressure easier - less social aid than for citizens, expulsion for non-citizens at every crime committed and legally convicted for, things like that.
Thats the problem with culture and identity, I admit. Its very vague and blurry at the edges. What Id do is as said to differentiate between citizens and non-citizens, and then make getting German citizenship be dependant on enough tests and ceremonial stuff to weed out people believing in "Turks with a German pass" and stuff like that. But since asked directly about language, yes, I would expect of every German (in the national sense) to use German as primary language.
Citizenship should reflect national identity, yes. How one defines that for oneself can vary - obviously nations are always in a constant process of assimilating cultural elements of other nations, anyway. Thats why I said the borders between "cultures" can be blurry. I really do not think Germany should (further) build up a parallel Turkish society inside Germany. Primary use of Turkish would be an indication of that. To vary my question from before, if they want to be in Germany, why do they use Turkish?
Because "operating" and "functioning" are not the points. Germany is the country of the German nation, and as such should be home to the German nation, and not to other nations (with the well-justified exceptions for traditional minorities, of course). Now, its possible to enter, so to say, the German nation - but that really is the point then and not functionality. Now, of course I say nothing against knowing Turkish, or even against being German-Turkish bilingual. What I said was about, well, judgement in when to use it. In Germany, and as German, ones primary language should be German. Knowledge of other languages is of course always useful, I dont think its ever a negative to be able to speak more languages, but they shouldnt be the primary language in Germany.