Submit the error as an issue, I will regularly check the issue tracker.
If you want to modify the existing .lang file or add a new one, you can directly initiate a pull request, and I will manually add it into METS.
Why the game crashed after adding METS?
Make sure that your IC2 version is 2-2.8.188-ex112 or between 2.8.194 and 2.8.199.
Will METS support lower / higher versions of Minecraft?
If IC2 releases a version supporting higher version of Minecraft in the future, METS will follow up.
For more questions about IC2 addons, please check about:
https://forum.industrial-craft.net/board/56-addons/