If I'm being totally honest, Sigil's spell-checking in general is completely wasted on me (one language or multiple). I'm just not a proof-reader. And if I was, Sigil probably wouldn't be my proofing tool. My usage tends to be with content that is already finalized. It's not that I don't care if the source has misspellings, it's that .... no, it really
is that I don't care. Fixing that stuff is someone else's job. I'm the nuts and bolts guy.
I realize it's an important feature for some, though, so I hope lots of people give their feedback on it.