I am wondering if I can use Turkish letters like ş, ü, ğ, ç, ö on my app?

Ooops, sorry. The letters didn't show on the forum either. By the way I found a solution, at least for now. I've found a website which converts whatever you type into standart HTML code, and the letters are now displayed correct after the conversion but only in custom HTML allowed menus.

You can search for forum for 'characters' and see lots of discussions about this. We've struggled for a very long time to get all the characters supported. We will eventually. It's mostly related to how the backend database stores this data. We'll end up migrating everyone to a newer, updated database eventually. We're hoping this solves the problem once and for all. In the meantime, you'll need to figure out work-arounds like you've already done.

thanks. can you give a date of upcoming updated database?(not exact, close date?)

Few weeks maybe? Trying hard to get it all done. The database is created and has been tested and does seem to work well. But, we've not been able to figure out the best way to 'migrate' everyone over. It's looking like we'll announce an 'outage' for a few hours in the next week or so while we move LOTS AND LOTS AND LOTS data so nobody is caught off guard. By the way, VERY cool to see Turkey in the forum ;-)

Thank you for your kind answer.