SDL: Fix invalid memory access in getSystemLanguage.

A call to setlocale can invalidate the string a previous setlocale call
returned. Instead of saving a pointer we copy the returned string now. This,
for example, fixes invalid memory access on my system.

See de8da01b0e8a309b9ed3f5b0f152ebbcf8f4af37 for the commit introducing the
invalid memory access.