Another translation of the motd/credits PR

Reverts the previous translation system and uses a file based system instead.

File names are formed by added a dot and the locale identifier, both version (with territory and without are tried). @VlasovVitaly Is that format acceptable? I didn't wanted to mess with the filenames to much (inserting the local somewhere in the middle), appending the local is easier.

If the local is cs_CZ.UTF-8, it will look for "data/motd.cs_CZ", "data/motd.cs", "data/motd" (in that order). The first file that exists will be used, same for the credits file.

The locale is either taken from the game options ("USE_LANG") or if that option is empty (system default) from the current locale.
Code::Blocks GCC didn't like extension as it's apparently not used enough.  Threw together a definition to get mine to compile.