3rd party mods translation

The game allows 3rd party mods to ship their own translation files for any language the game supports.

Below are simplified steps that most of the modders will be able to follow to set up translation workflow (this one is for Windows):

1. Install Poedit (https://poedit.net/download)
2. Install Python 3 (https://www.python.org/downloads/)
3. Open Windows CMD and type pip install polib
4. Copy extract_json_strings.py, dedup_pot_file.py and extract_mod_strings.bat from the lang/ folder of repository to the mod folder you wish to translate
5. After launching this .bat file there will be extracted_strings.pot in the mod's lang directory, which can be edited with Poedit (see instructions in TRANSLATING_MODS.md)

For further details, including instructions for MacOS and Linux, see TRANSLATING_MODS.md

PR that implements mod translations, for reference: https://github.com/cataclysmbnteam/Cataclysm-BN/pull/505

Example of the mod translation: https://github.com/Kenan2000/Bright-Nights-Kenan-Mod-Pack/pull/36