Also, there doesn't seem to be so much information about long-term meditation retreats in Europe. So, do you know any?
No, not outside my own tradition. And within my own tradition long courses are only offered to long-standing practitioners who meet a strict set of pre-requisites.
Asia comes to mind, of course. It's cheaper and there is probably an abundance of places to choose from. There might be visa issues however. Especially if staying for longer than 3 months.
If you can identify a centre or monastery that offers a long retreat and if they offer you a place or sponsorship, the country may provide you with a special long-term stay visa. For example, Burma offers meditation visas for people who are offered places at centres or monasteries to study Theravada Buddhism and Theravada Buddhist meditation. If its something you want to do, then look into it!