Sprint Verdict

Live demo, the full app on sample data, no install: here
Documentation, user guide, security and privacy: here
Runs on Atlassian: no servers, no database, no third-party services, no outbound network requests. Team configuration is written to a property on your own Jira project, so you can read it over the REST API, export it, and keep it if you uninstall.
Works with whatever you already estimate in: story points, dev days, hours, or issue count. Leave can be typed in or read from a Jira board you already use, and public holidays are kept per team.
Reads are filtered to the projects you can browse; every write needs project admin, checked against Jira on each request.
Interface language: English.
A new listing from an independent developer, so no testimonials yet. Report a problem and you are talking to the person who wrote the code.