Peter's comment indicates he feels 'were' is the preferred phrasing, and so do I.
'behave as if it were' is meant to indicate it is counter-factual (substitute 'even though it's not') so the subjunctive should be used.
Also it just sounds better that way to me.
I'm hesitant to disagree with @Barrie-England given his credentials, but I don't believe "were" would be grammatical in this case. As he mentioned, "it conveys varying degrees of remoteness from factuality." In your example, it's unlikely that you're asking your reader about his or her wishes, opinion, judgement, or possible state. Your statement is asking for clarification of a definite and factual characteristic.
Answering the question with a "were,"
The interviewer asked me if I were fluent in English.
would leave the audience (me at least) wondering what the uncertain event, action, or state would be. Expanding it to something like the following to add the necessary element of uncertainty would seem grammatical to me.
The interviewer asked me if I were fluent in English, would I then travel to England.
That's probably an awful example, but I see "were" being used with words that denote uncertainty: would, if, should, could, wish, must, etc.
I see "was" being used to denote certainty. Presuming your question was expected to be answered with a "Yes" or a "No," "was" should be used.
Really, though, shouldn't they have responded with:
The interviewer asked me if I am fluent in English.
Best Answer
In third-person past-tense singular subjunctive, you use were to describe an unreal or imaginary scenario that didn't happen in the past. You might say,
But since in your sentence you're just using third-person past-tense singular, non-subjunctive, you should use was, because the example is not a hypothetical or unreal scenario, it is merely unknown.
I recommend inspecting the fascinatingly convoluted definition of be, to see how it's used in different tenses and forms:
http://www.dictionary.com/browse/be?s=t