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Language By Path 1.1

Loads a language based on the path.

Vanilla Forums by default is English-only. You can install locale packs to make it something-else-only. You can also install Multilingual, which lets users choose which language they see.

I believe the best way to handle localization is to have different languages at different URLs. That's what this plugin does. /en/forum/ will load the forum in English, /fr/forum/ will load it in French.

Installation instructions.