Flow-typed JavaScript works well with our existing JavaScript support. Post an issue if you have any issues when trying it out: https://github.com/sourcegraph/issues/issues.

For Ruby, we haven't investigated the situation in a few months. Last time, we checked out https://github.com/mtsmfm/language_server-ruby but it was not far enough along. We maintain a table of language servers at http://langserver.org/, and once they have support for hovers, definitions, and references, we begin to plan to integrate them. We'll be releasing a roadmap for Sourcegraph language support soon with a lot more docs for how to integrate existing language servers, too.