Likely /ˈdaɪ.mənd/. For reference I pronounce “diamond” as /ˈdaɪ.ə.mənd/.

Typically English stresses the first syllable, so my initial instinct would be to stress the “dy”. And stressed “y” is generally rendered as /aɪ/, as in “by”, “fly” etc.

Diamond

Dye-mahnd

If I saw that in an English-speaking area I would assume it was someone getting creative with the spelling of Diamond and pronounce it the same, specifically the two-syllable pronunciation: DI-mund.

Di-Mo-Nnnnd

Demon’d

dim mund

What language and what use?

If it’s a name in English, it would probably be die-mond, dee-mond, or duhmund (without a distinct verbal gap between syllables), based on when I’ve heard it here in the south, or up north during visits.

That’s the only context I’ve ever seen it in, as a surname, and not a super common one. I’m not aware of what language it stems from.

As a side note, since you’re asking about speech and language, your title “how would you pronounced dymond” isn’t standard grammar. It would just be “how do you pronounce?”. Doesn’t matter, we all know what you are asking, just a bit of info.

Back to dymond.

By the spelling of it, I can’t see it being pronounced “demon” in English. That final d would be pronounced almost certainly if it isn’t a name. If it is a name, it could be that, since a lot of surnames and even other names have origins in other languages.

However, we sometimes pronounce diamond without the a being said. It comes out dimond, with the I part being said in different ways. There’s a little town in the next county over where it’s said more like “dah-mund”, or “dai-mund”. Similarly, there’s three syllables in my town, but it’s “die-yuh-mund”

As another aside, one of my aunts is from that other county, and their local accent is weird. They say yes with two syllables: yay-us. Well is way-ul. They’d reply to being asked if they are thirsty with “yay-us, git me a drank from the way-ul, if’n y’don’t care t’.” Which, even if you’re a native English speaker is a bit of a mind fuck, and they tend to have this way of speaking where their lips hardly move sometimes. So, you know, good luck having a conversation if you go there lol.