Child thinks that the German language is a different one. Are these signs of something?

My nephew is 5 years old and really enjoys watching his children's series. He mainly watches them in German. He himself is half German and half Turkish. He understands Turkish, but he can't really speak it.

For the last few days he's been saying the series (which I turn on for him on Netflix) is in English. Of course the German language is set, but he is of the firm opinion that it is English and he does not understand it. If I set the whole thing to Turkish, he recognizes the Turkish language. But he only recognizes the English and German language as English. That's pretty strange and funny too, because of course he doesn't speak English yet.

Now we don't know if he's just looking for attention or if his behavior is indicative of something?

By the time he is 5 years old he should be able to at least understand and answer the language you use to speak to him at home. If he doesn't understand English at all, he doesn't know what he's "saying"; that's just how children are. Why do you let him watch English programs when he's already overwhelmed with two languages?

I would rather worry because he can neither speak Turkish nor German without a problem.

At that age, my nephews spoke their mother tongue (German at home) and English (with the neighboring children and at school) without an accent.

I didn't say we'd let him watch programs in English. It's best to read my question again, think you got it wrong. He speaks fluent German without an accent. He also understands Turkish, but doesn't speak it.
Aaaber thinks that the German episode we turn on is in English.

"but he only recognizes the German and English languages as English."

Then he has to hear English at some point, otherwise he wouldn't even know English or what it sounds like.

"… But he is of the firm opinion that this is English and he does not understand it"

Well, what now, does he understand the programs he is watching at the age of 5 or does he not understand them?

No. Again. He doesn't speak English so it's strange for us.
He knows that there's such a thing as the English language, but he can't speak that language. And he thinks the German program is in English.

Therefore, by "he recognizes it as the English language" I didn't mean that he understood it, but that it was English for him, he didn't understand this language and we should please switch to German.