In the owner’s profile, I changed the system language from "German" to "English (Germany)."
After that, the Play Store updated some apps with language updates. Then, it asked me to restart the Play Store to update it, which I did.

It gave me a warning that said:
"Update this app from Play Store?
This app normally receives updates from the App Store. By updating from a different source, you may receive future updates from any source on your phone. App functionality may change."

Has anyone else had this issue, and is it a problem?

I have the latest Stable Update installed (2025030800)

I accepted the warning and there was no further problem.
But I don't know if this can lead to problems.

Should I reinstall the Play Store via the GrapheneOS App Store?

    sh4tr0x

    This shouldn't be an issue.

    I do not advise reinstalling it, as this will break your Play Services install and require that you also reinstall every app that depends on Play Services as well.