Generally default uppercase/lowercase implementation in class libraries/frameworks respect active locale’s behavior. Due to difference in handling of letter I & i  applications may fail when locale-aware case conversion is not desired such as parsers, generators etc. The general fix for this issue is to use an English locale in case conversion. I’ll give a few example bug reports.
- A bug in java-gnome bindings generator I reported and its fix.
- A Turkish locale specific bug in JAXB .
- A bug I hit in Bazaar that hinders my bundle generation, merge process which only happens in branches with signed commits.
- A bug in Exaile media player
I guess, I can give more if I google, but I think the above examples are enough to show why applications fail with Turkish locale.
Addition: I traced Bazaar bug to be really a bug in Python itself, see: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=489740 for how Pardus and Debian fixed Turkish locale issues. Thanks to Jesus Rivero for quick follow up after my request of applying these patches in Gentoo. Hopefully they’ll soon be incorporated in portage.
Further Addition: Fixes are committed to portage by Ali Polatel. See bug #250075.