Trailers for Store content (buy/rent movies) are prepared by the supplying studios (Universal, Warner Bros, A24, etc). That is not something that Apple decides on for your country. In multilingual countries, sometimes the studios make a choice for one or the other language per Store item. In some countries (e.g. Swiss, Canada), multiple language versions may be available as separate Store items.
The actual Store movie may have different language options than the trailer. See the textual details under the Languages header for each movie.
You may give feedback if you think that Apple should get more involved in the offered languages, in a way that meets your needs (for your country). Feedback - Apple TV App - Apple.