I googled and was unable to find a satisfactory English translation, so here's my attempt, after copious dictionary usage, with a focus on accurate translation rather than meter and rhyme (at least for now).
When the nights get longer And the cold sets in Thus says the small mouse mother To the young nest of hers: "If noone goes in the mousetrap, But is fit for it anyway, Shall we all together soon Get to celebrate Yule again!"
(chorus) Moshing and leaping and falling all again 'Round Yule evening then Shall everyone be glad! Moshing and leaping and falling all again 'Round Yule evening then Shall everyone be glad!
Yes, mouse mother is hard-working, She takes a piece of coal And blackens the ceiling and walls In their little mouse hole, While
The first line of the chorus includes some made-up words, so I'm improvising based on the roots of the words.
I will not include the Norwegian version since it's still under copyright and there has apparently been some conflict about that already (greedy lawyers games I suppose), but I assert that this is my own attempt at a translation, so it is copyright by me, Shawn Rutledge, 2011, and it is free to use under the creative commons license.
<a rel="license" href="http://creativecommons.org/licenses/by-sa/3.0/"><img alt="Creative Commons License" style="border-width:0" src="http://i.creativecommons.org/l/by-sa/3.0/88x31.png" /></a>
English translation of Musevisa lyrics by <a xmlns:cc="http://creativecommons.org/ns#" href="ecloud.org" property="cc:attributionName" rel="cc:attributionURL">Shawn Rutledge</a> is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-sa/3.0/">Creative Commons Attribution-ShareAlike 3.0 Unported License</a>.
Based on a work at <a xmlns:dct="http://purl.org/dc/terms/" href="http://ecloud.org/index.php?title=Musevisa" rel="dct:source">ecloud.org</a>.