The courses are "officially" given in English and Italian (everything is explained in both languages), but German is also possible (Elke Ahrenholz who teaches most of the course is German) and even a bit of French is also possible, but Elke is not so good at it. Dutch, alas, no. When I attended Tier 1 almost everything was explained in English, Italian, German and French and luckily I know enough of all of them to get the explanations four times.
Castelfidardo is pretty boring for a partner with no accordion interest. There is a park with the statue of the region (a war memorial) and apart from that you only keep yourself occupied by eating lots of gelato. There are villages and cities around which may offer enough entertainment if you are by car. Nothing is within walking distance of Castelfidardo.
Options to stay are either Hotel Parco (in the former premises of Crucianelli) and there are several B&B's, including Covo dei Piccioni at just 50m of Victoria, where the courses are taught. While the courses are given in English (and Italian) you should not expect staff in stores, B&B's, restaurants... to know much English. You are in Italy, and people only speak Italian. (Young people do speak English, but older people, alas, no.)