In Dutch, there is this expression which I would translate to 'this is clear language'. It means that something is very clear and that you can't argue with it. It's often bluntly spoken with authority.

Is this sentence 'This is clear language' a good translation, or is there another way of saying the same thing which might sound more natural/common/better/etc...
It sounds a bit unnatural to me now.

I've never heard "this is clear language"... I could suggest "this is crystal clear", or "am I making myself clear?". I'm sure others will come and help.
Given that you said it's often spoken bluntly and with authority, you could say "And that's the end of the discussion." Or "There's no more room for disucssion." But that clearly comes across as a superior to a junior.
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"And that's it" ?
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Another possibility:

That was plain speaking.