I suggest using a single string and then call it in loop to translate multiple strings. It’s because of the limitation of AdaptiveMtTranslation of handling a single string in the content field. Though it’s not directly indicated in the documents, error 400 is about the number of entries, which should be 1 only.

On Google side, there is a ‘feature request’ that you can file but there is no timeline on when it can be done. You can request this so that they can check if they can update to support multiple strings without calling a single string in loop.