If you have a list of forums you know, one great way to find out which extensions are used is the Lab https://discuss.flarum.org/d/10056-migratetoflarum-lab-the-health-scanner-for-flarum
Also, any reason to translate using a language pack and not in the extension itself ? We gladly accept translations directly on Flagrow packages for example.
One big advantage I see in translating at the extension level instead of the language pack level is that you can correctly match the translation with an extension version. If we release a breaking change that not everybody can install (Flarum core version, PHP version requirement...) and that is using different strings we can have both versions accurately translated. That might not be possible with a language pack, that has to cover every single available version of a package at the same time ?
Also I made a feature on the Lab to find out the state of translation of every extension, maybe it will be useful to translators. It's far from complete, don't hesitate to give feedback on it.