It is actually duplicate of QTCREATORBUG-4370, but I thought I should attract some attention now that root of problem is found.

Initial problem is, on non-standard (i.e. non-english) locales gcc search paths are not found. So, no code-completion for stl etc.

Root of problem lies in the fact that gcc uses l10n for some output, which is parsed by qt creator. Namely, line.startsWith("#include") in gccHeaderPathes from plugins/projectexplorer/gcctoolchain.cpp does not work for some locales.

The most direct approach to solving this problem is adding LC_ALL=C to environment when executing gcc. I conclude that same is true for every other call to gcc in project explorer. Proposed patch attached (ugly hack, really, but it works with minimal set of changes)