Hey, I don't think it's uncommon to have a previous knowledge of the language and there are usually two ways to go about the degree once you're there:
(1) If you do have a beyond-basic level of your chosen language, then you might be placed in the Intermediate (i.e. second year's) class for your first year. I don't know how all the examinations work but I believe that you then take the intermediate course again with your peers the following year.
(2) If you're language isn't quite good enough, or that you and your tutor agree that option 1 isn't the best or that the beauocracy doesn't allow it, then you'll just go into the basic (i.e. first year's) class in your first year and risk being a bit bored for the first term or so.
Therefore, though I wouldn't discourage learning the language beforehand, there isn't a massive advantage in doing so, though by the time of the examination period you might be more confident in the language than your peers.
However, I'm not 100% sure on the ins and outs of this and you're better contacting the actual department:
[email protected] is the admissions HQ for the European school
If you go onto ucl.ac.uk/scandinavian-studies or ucl.ac.uk/selcs, you might be able to find a more relevant contact under undergraduate/staff/department etc. links.
Are you just doing straight Scandinavian Studies??