The GJ 3.0.1 nightly release has fixes the stable doesn't have. It's recommended to upgrade if you want to remain most up to date. If you're using 3rd party language plugins it could also be an issue with that language plugin being incomplete as well.

Our nightly releases are not beta releases. We do have beta nightlies sometimes, but they're marked as such. Our nightly releases are nightlies of our stable releases. They typically include minor bug fixes or feature implementations (see our forge for 100% items, those will be in the nightly). In short our nightly, IMO, is more stable than stable.

We'll be redoing our filename version format when we launch our new site to be more semver compliant so nightlies will be easier to identify.

GJ comes with its own default language files. If you do not have a language plugin it will load its strings from those defaults. You need a language plugin in order to translate GJ without having to modify the defaults or attempt to use a language override plugin (note this isn't very reliable as due to plugin load order it may not work at all).

See the below and the CB 2.0 Primer Book for language string usage information.

www.joomlapolis.com/cb-solutions/languages

We do have plans for implementing a backend interface for adding overrides or new language strings in a future CB release.

Problem with the override plugin is it's just a plugin that's loading its default language files. Beyond that there's nothing special about it. It's not a specific language plugin case that CB even handles. This means plugin load order can really mess up whether it works or not. Once we've official override support implemented that won't be an issue and such plugin won't even be needed. For now try changing the order over your override plugin to be after GJ and see if that helps.