"I sleep beside the window."

Translation: Az ablak mellett alszom.

[KatjaJuliannova]

'En alszom az ablak mellett' is allowed but not 'Alszom az ablak mellett'. Is the 'En' necessary if I use that word order?

[vvsey]

"Alszom az ablak mellett." is a fine sentence, but it gives me the sense of the present continuous: "I am sleeping beside the window." If somebody asks "What are you doing?", this is a good answer.
If there is an "Én", then a question could be "Who sleeps (or is sleeping) beside the window?".
Anyway, I guess your version is also acceptable.

[TamsMar]

I agree. I think it may be because it's only in beta.