I’ve experienced the non-English thing in a few different contexts and while I love the diversity of languages and speak a few myself, I think not mandating an official language to be used in the office is a mistake.

Your case is the obvious one: if most people speak Finnish but you only require English of your hires, sooner or later you will have people unfairly excluded from the social part of the office. Even if you’re in Helsinki, you need to either require that work conversations be in English, or make obtaining Finnish fluency an official part of the job. The latter being the much harder route.

However, in real life this requires a more enlightened C-suite than you probably have (cf. butts-in-seats order), so you might consider carving out an hour a day to quietly learn whatever language is being spoken by the rest of the team. It’s probably easier than Finnish and it could help your career!