How to change "theorem" to what ever it is called in my own language?

[A B]

I started to write with document class= article and language =
swedish. I then activated the module "Theorem" and added a theorem and
a proof.
In the lyx window it shows "Theorem" and "Proof" (should it not be in swedish?
But I can live with that lost translation, but the postscript preview
shows only "Proof" to be translated. "Theorem" is not. So how can I
fix this? Or have I done something wrong?