Learn Finnish: Oliko hänen äidinkielensä englanti?

We use the translative case (-ksi) when expressing in what language communication happens (for example, "Sano se englanniksi" - "Say it in English"). In this sentence, "English" isn't how someone is communicating; it's simply the name of the language acting as a noun defining the subject. Therefore, it stays in the basic nominative case, englanti.

Why does the sentence use both hänen and the -nsä ending on äidinkielensä?

In standard written Finnish, third-person possession is "double-marked". If you use the genitive pronoun hänen (his/her) or heidän (their), you must also add the matching possessive suffix (-nsa or -nsä) to the noun. In casual spoken Finnish, people usually drop the suffix and just say sen äidinkieli.

Is äidinkieli literally translated as "mother tongue"?

Yes! It is a compound word made of äidin (mother's, the genitive form of äiti) and kieli (which means both "language" and the physical "tongue").

Why does the sentence start with oliko?

To form a yes/no question in Finnish, you add the question particle -ko or -kö to the word you are focusing the question on, and you move that word to the very front of the sentence. Here, the core question is "Was it?", so the verb oli (was) gets the -ko particle.