Hi,
It's pretty cool to have a language for an A level, however it's not necessary with the number of subjects you are taking. If you are looking to go to university, they only require 3 A levels and having any extra is not considered. If anything, taking on a fifth A level is just spreading yourself really thin, which might affect your academic performance in the other subjects.
Personally, I wouldn't take on another A level even if it is a language one because you have a strong set of grades from the subjects you are doing. Just focus on those and get the best grades you can, despite Turkish being your first language. Also, if it's your first language you don't have to prove to others that you can speak, read and write it with a qualification. When you fill in applications and go to interviews it could be something you mention as a skill, but honestly, I wouldn't recommend taking on a fifth A level.
Sorry for rambling but I hope that was helpful!!!