Custom or altered language files are not loaded through resource packs. I've tried checking around to just see if I was doing something wrong, but textures and sounds are working fine, so I don't think I am.
The directory I place the file into is packName\assets\minecraft\lang
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My apologies. I'll try searching for it a bit more next time.
Duplicate of MC-17840, please use the search function to see if your bug has already been submitted. Currently over 57% of tickets are being closed as duplicate.