I don't know about the arabic language pack, but I can make the following general points:
- Hundreds of registered Moodle sites in the middle east (https://stats.moodle.org/), and that includes some big players like the Arab OU.
- Moodle has many features, most of which you won't use. And most features have obscure error messages. 72% of all strings translates can easily be 99.9% of strings anyone ever sees translated.
- If you hit an un-translated string that is really important for your Moodle site, you can immediately fix it in your own site using Language customisation - so you won't be stuck waiting for someone else to translate it.
- And, translating Moodle is a colaborative effort, so as well as fixing the translations yourself, you can contribute them back to the main project at https://lang.moodle.org/.
Regarding how this is all configured, and how it looks to users, the best thing is to get a test site, and experiment for yourself. You can start experimenting at the demo site https://qa.moodledemo.net/ which seems to have lots of languages installed. You can also read the docs at Language settings.