It's not a bad joke because it would be too soon, but because Dovahzul has far less redundant words than English and in many cases a different syntax structure. "I am eternal" and "I eternal" are semantically equal - there is no difference in meaning. The "am" is still obligatory in English because English syntax does not allow for a sentence without a verb (so we fill the gap with an auxiliary verb that now has no meaning in and of itself in the given context) - Dovahzul on the other hand does not share such grammatical constrictions.

But http://thuum.org/learn/grammar/nouns.php says it better than me.

Depending on the context you might not need a helping verb. Dovahzul doesn't have contractions like "I'll" or "I'm" so if you wanted a literal translation you'd have to split it up like "I will" or "I am". Here you might want to keep "will" because "I have it cut from your head" means something rather different than "I will have it cut from your head".

The cool (or challenging!) thing about translating songs is that you can play with the lyrics to make them fit the language and the verse. "Watch your tongue, I'll have it cut from your head" would literally translate to "Lingraav hin sahkren, zu'u fen lost nii vey nol hin klov." Unless you can speak really fast, it's going to be rough to make this work!

So, perhaps we switch things up and translate this line to "Lingraav sahkren uv lost nii vey nol hin klov". This literally means "Watch (the) tongue or have it cut from your head". It's close to the original and fits to the rhythm.

Thank you,Paarthurnax.Now the only problem I'll be facing is everytime I defeat Alduin,I'll be thinking of him saying "I eternal!I cannot die!".Surely now you can say you're happy;he had the grammer of a modern day high schooler.

I'm sorry,it's much too soon to joke like that.