I’m not sure whether this sentence refers to past or present situation:
You couldn’t do the job if you didn't speak Japanese fluently.
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This is called a conditional sentence. Here's how Wikipedia (Conditional sentence) defines it:
Conditional sentences are sentences expressing factual implications, or hypothetical situations and their consequences.
I think what you've got there is know as the type 2 conditional sentence. While it's true that the sentence uses past-tense grammar, it itself does not necessarily have to refer to a past-tense situation. It could, but doesn't have to. Strictly speaking, it describes a situation that's hypothetical in some sense. Take a look at these examples:
If you went to bed earlier, you would not be so tired.
Since that condition is not true because you did not go to bed earlier, the implication is that you are very tired right now.
In the past, you couldn't work in Italy if you didn't speak Italian.
This sentence refers to a situation that was true in the past (if you did not speak Italian, you could not work there), but at the same time it implies that this is no longer true today (if you don't speak Italian, you can still find work there).
You couldn't do the job if you didn't speak Japanese fluently.
Assuming that you're talking about a particular person doing a particular job and you in the sentence is not the impersonal you, this condition implies that the person you're talking about did the job because they were able to speak Japanese fluently. And that obviously refers to a situation that happened in the past. So, it was possible for them to do the job because they could speak Japanese fluently.