The easiest solution would be to recommend a telepath, especially particularly intelligent ones such as members of the abra line, as they can “cut out the middle man,” as it were, by projecting what you and your boyfriend’s mother mean into each other’s minds in a way that the both of you can understand. However, it would be far, far more productive to use a pokémon that can both translate verbally and act as a teaching aid.
To that end, chatot and murkrow tend to be popular among those who are learning new languages, as they can remember simple phrases and help their human partners recall past lessons or match concepts to new vocabulary. In your case, you could, for example, teach a chatot a word or phrase, connect it to a concrete concept, and have chatot teach your boyfriend’s mother or vice-versa. Alternatively, that chatot can serve as a translator if either of you teach it your own language in your own time—or at least enough to translate simple conversations.