Been using English since they days before bunq introduced support for German. Not sure which way would be the best, but just some thoughts / options that came to my mind:
1. Try to assign / "match" support person depending on the language that is used in the app (like you're already using the app's - or in effect phone's - language preference for personalization of payment cards)
2. Don't limit to English and Dutch. bunq is actively targeting Germany/Austria, Italy and Spain. Keeping in mind that we're dealing with technical terms here, even users with advanced English might not be familiar with all banking/payments terms. Support interaction could be much more efficient if you have a "match" in a third language.
3. Have a setting for customer's language preferences in customer profile. Maybe that'd be "better" / more efficient in some cases than just defaulting to English. For instance, Spanish language speakers might be more proficient in Italian and vice versa, as opposed to English, due to higher degree of mutual intelligibility between these two languages, especially in writing (even higher for Spanish and Portuguese). Luxembourgish speakers will often be more well-versed in both French or German than English, etc...