Please double-check that your CSS rules are not present in the page using Chrome inspector or Firebug. You don't need any extra work - just putting the file in should work. See how Terminal plugin does that: https://github.com/Eugeny/ajenti/blob/master/plugins/iptables/files/fw.less

Yes, you can receive JSON monitoring data at /api/meters and /api/health endpoints (HTTP GET).

If authentication is enabled, you need to first make a HTTP POST to /auth with parameters username and response (which is password). HTTP response header X-Ajenti-Auth indicates auth status. For the root page, you will receive X-Ajenti-Auth: start if auth is required. For successful /auth request, you'll receive X-Ajenti-Auth: ok

This is already implemented in 1.x series source. Stay tuned!

It looks like you installed Munin plugin but didn't configure it properly. Please check if Munin URL and credentials are right under Configuration > Plugins > MuninClient

The problem here is not to get strings translated but to get multi-language support in Ajenti itself.