Added lang file definition checker to DI\Extension

If the definition of language files is missing, or if the extension is not defined correcly, a proper exception should be thrown from a compiler extension.

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Also I have found some kind of a bug while trying to edit your files and send a direct pull-request via github website. I wasn't able to do it so I had to do it by pull-requesting from your repo to my repo and then update it and send a new pull-request to yours :D