What do you mean by "extensions are there that can be installed locally"? Depends on what mail client you are using. Thunderbird can do this. I use mu with emacs mu4e and it just works. You need gpg installed, that's all.
FlowCrypt operates as a Chrome extension, so I assumed that the email client in question was the Gmail web client (or Inbox by Gmail). One can install extensions from the local filesystem rather than the Chrome store, which removes the threat of autoupdating into adware or malware. However, I don't know of any open source crypto extensions built to be installed that way. FlowCrypt might work, but the licensing terms don't seem amenable to that approach.