Hello Markus,
SAP payroll for Turkey is being used by numerous companies.
In Turkey payroll processes are really different from Europe. For example in Europe payroll is important but year end processes are really big issue. But in Turkey there is no year end process.
Every calculation is generally done in monthly payroll. There are some legal restrictions for Turkey. For example normally you can run retro payroll for countries but in Turkey you can not run retro payroll unless you are a member of a union and you renew your union agreement.
Also Turkish localization does not support off-cycle payroll.
Most needs for payroll is covered by SAP Turkey localization team but in some cases which is sector specific etc SAP Turkey localization team inform you that that is not a scope of Turkish localization. In some of our projects we had been informed that being legal does not mean that it will all be supported by Turkish Localization but again it is not so common.
So for Turkish payroll it is always advisable to work with a local consultant who knows Turkish payroll procedures. We have been involved in some projects where we did not make any customization but have leaded the payroll consultant in terms of how to do certain things in the system. Wiki which Siva has sent you is also a project of mine, I have tried to write there but unfortunately could not widened it that much.
If you need any assistance please feel free to ask.
Regards