I need to type special characters (ä, ü, ö, ß) quite often. Normally on MacOS applications this is done by press-and-hold a key and then select the special character. For applications where this gets in the way (e.g. MacVim) this can be disabled.
TextMate 2 seems to have this feature disabled by default. I can't figure out how to enable it. The command 'defaults write com.macromates.TextMate.preview ApplePressAndHoldEnabled -bool true' does not work.
I assume you are on an english keyboard layout? If so, you might want to consider to activate the "U.S. International - PC" keyboard as input source for OSX. You can write ä,ö,ü as " followed by a/o/e (other accented letters can be created in a similar way) and ß is simply alt+s. Works very smooth and fast for me, I could never go back to a german keyboard layout.
/Carsten