Used to be Russian. The company moved to Cyprus.

Most of their software (including AdGuard for Safari and AdGuard Home) is open source, so there's little chance of anything nefarious happening.

Except there's no easy way to verify that what you get from the store has been built, unmodified, from the public source - afaik.

(I still use AdGuard fwiw)

AdGuard Home has no "store". You download it yourself. I believe the same is possible for the Safari plugin - they're not required to be obtained from the App Store.