Lang support must be required for all plugins with backend/frontend user interaction.

A good example of how language can be implemented is in the translate plugin:

https://github.com/rainlab/translate-plugin

I'm not sure if this should be made mandatory for authors as it involves a significant amount of overhead.

Sorry for my english. I think my request was not understood.
I wanted to refer to the language of each plugin. If I want to make a website in a language other than English, there are plugins that not use /lang/en/lang.php files. And you can not sell a job with mixed languages in backend.

I hope my request is understood now.
Thanks.

There is a large overhead in creating plugins that are multilingual, so it should not be mandatory. You can suggest it to authors or even submit Pull Requests as most are open source.

Thanks @daftspunk for your reply.