Translation loader now ignores system files

I was having an issue with habitrpg failing to load when OSX created a
.DS_Store file in the locales directory. This code fixes that by
ignoring all hidden files (files that start with “.”)
Locale loader now ignores non-directories

Locales must be stored in directories and the code previously crashed
if there were any unexpected files in the locale folder. Now the code
simply ignores all non-directories.
Only load .json files from locale directories

This opens up the possibility of putting READMEs or what have you
inside locale directories, and will avoid problems if system files or
folders end up in the locales.
Merge pull request #3527 from OverloadUT/locale.ignore.system.files

Translation loader now ignores non-directories