Deutsch ist kein problem, but I'll reply in English.
I've tried all the options I could find, but found none to be easy enough to work with. Letter combo's like , +c and : +o or Alt number combinations I don't like. The Alt+letter works perfectly for me.
ç ö ü are not the problem as they are also in the latin character set. ş ı ğ are the only available in unicode (/other character sets), so the first letters you'll be able to find on many keyboards & character sets, the last more difficult.
I'm using a US int keyboard layout, so there none of the letters are available by default, German or Dutch keyboards have some, but not all. This little tool should work for all keyboards though.