Soulver 3 requires macOS 10.14.4 or later. If you're technically inclined, you might note that this is the first version of macOS that had Swift built right into the system. Soulver is written in Swift, and we didn't particularly want to ship Swift's runtime inside Soulver's bundle.
If you're not running Mojave or later yet, you can still get Soulver 2 (which supports macOS 10.10 or later) on the Mac App Store.
Right now Soulver 3 is only available on macOS. You can still download Soulver 2 for iOS on the App Store. On Windows you could try OpalCalc, which was inspired by Soulver (with attribution). You can probably find a reasonable copycat on Android too.
No, it's a separate purchase. Note that it's not a subscription purchase, so it's a pretty great deal. Historically we've also been pretty generous in terms of free updates. Soulver 1 had five years of free updates, and Soulver 2 had nine years of free updates.
Not by default. Soulver 3 stores your sheets inside a "sheetbook" file in its application support folder on your Mac. If you'd like to sync between multiple Macs, you can move the sheetbook to Dropbox or iCloud, in Soulver's General preferences. Please note: Soulver 3 for Mac cannot sync with Soulver 2 for iOS, as they use different file formats and math engines. Personally we continue to use Soulver 2 for Mac (for when we need syncing with iOS) alongside Soulver 3 on the same Mac.
No. Soulver does not analyse complicated paragraphs of text for what you might mean. Rather, it uses simple and natural human syntaxes to invoke helpful functions. You can see what particular syntaxes are supported in the syntax reference.
Not right at the moment, but we intend to roll out more languages later this year.