Google Assistant's language support was expanding quickly toward the end of 2018, but since the beginning of this year, the rate of additions slowed down considerably. As Google I/O nears, we're sure to see those pick up pace a little. Case in point, Assistant now supports Arabic (Egypt) and (Saudi Arabia). The former appears to be in open beta and available to all, while the latter is in closed beta and only showing for a few people.
Head over to your Google Assistant settings, then the Assistant tab, Languages, and pick Arabic in the language selector. You can set it as part of a bilingual choice, along with English for example. Or, if your device is set to the same Arabic dialect, you can simply trigger Assistant by tapping and holding on the Home button, and it will launch.
Most of Assistant's smarts work. The dashboard with your agenda and contextual cards has been translated, same with the most important answers (weather, sports, general info, jokes, animal sounds, etc...). However, smart home commands don't seem to work, and I can't get my Google Home Mini to understand a single word. So support appears to be limited to phones for now, which makes sense as Home doesn't add a language until it actually launches in that country.
Arabic support for Assistant hasn't been officially announced yet, and it clearly says it's still a beta once you choose it, so keep that in mind. In my experience, some of the pronunciations and words are a little weird.