I am almost at the end of writing my Ph.D. thesis. I study in Germany, the language of the work is English and my mother tongue is Turkish. The doctoral study was a result of an international project, in which Turkish cooperation partners have also participated. I want to write the abstract section in 3 languages (English/German/Turkish), as I can reach a wider community thereby.
There is also an emotional part. My Ph.D. thesis is the most important work of my life till now, I want it to have a part in my own language. Is it allowed at all?
Whether it's 'allowed' or not, formally speaking, is exclusively the decision of your institution. However, this is your thesis, and within reasonable parameters you can and should do with it what you feel is right. Adding an abstract in your own language is certainly well within the field of reasonable.
In particular, it's important to note that many universities in Germany actually force you to write an abstract in German even if the thesis is in English (example), so adding an abstract in German is simply part of a standard practice in the country. Adding an abstract in a third language, which only adds value to the thesis without subtracting anything, is not that far from that.
That said, to be safe, you should still check with your institution - the library, or the director of postgraduate studies, or whoever is in charge of thesis submissions - that this is OK. If they say no, then you can still try to negotiate your way around it (leveraging your supervisor and other staff if necessary), but it's unlikely that they'll refuse.