The CE mark has been mandatory for windows for about 10 years. It documents to the customer the compliance of the product with all requirements of the applicable European product standards or approval specifications. No CE = rubbish
May I ask how you came up with the absurd idea of sourcing windows from Turkey? I have never heard of that and cannot imagine that there are reasonably priced options for that. Additionally, there is the higher Turkish VAT and possibly customs conditions (import turnover tax and whatever else).
Every "normal" homebuilder in Germany, if they want to really save money, buys windows in Poland – but not in Turkey.
I specifically took a window fitter with me to Turkey; he has been trained for 5 years and is well versed in the subject. We looked at the profiles, glass, fittings.
The window fitter was also very surprised and said that we MUST buy the windows at the offered price!!
GU, HOPPE installed 3-layer glazing, Uw=0.85 84mm profiles, 7 chambers, 4-12-4-12-4
We also looked at the production, everything equipped with new machines. I bought my windows at one-third of the price