Called once initially and every time gmod_language is changed.
This will load & fill SlashCo.LangTable & if missing, set SlashCo.LangTableFallback
It first always loads the gamemode file slashco/lang/[lang].lua and then loads any addon languages by searching slashco/lang/*/[lang].lua using SlashCo.LoadFileFromAddons
And if SlashCo.CurrentLang is outdated (indicating a language change), it will trigger SlashCo:LanguageChanged