language should be supported in atom feeds

[eikowagenknecht]

According to the spec (https://validator.w3.org/feed/docs/atom.html), language can be supported using xml:lang. Unfortunately, this project here does only support it for RSS 2.0 feeds apparently.

[jpmonette]

@eikowagenknecht I welcome your contribution to the package if this is a priority for you, please feel free to create a PR.

[syhily]

Any updates. I thinks this feature is quite usefull.

[mmilanovic4]

I see #226 is open and tackles this along with some broader changes (per-item language, JSON Feed 1.1 bump) but seems to have an outstanding review question that hasn't been addressed.

If it'd help, I put together a smaller, focused patch that just adds xml:lang to the root <feed> element (matching how language already works for RSS 2.0) — happy to open that as a separate PR if you'd rather land something narrower first, or happy to leave it to #226 if that's further along. Let me know which you'd prefer.