What would you prefer as a place to communicate with each other?
- Matrix channel on matrix.org
- Matrix server with multiple channels
- Mattermost instance

I use Mattermost and it's fine for multiple/large teams. However, for a community project, I suggest looking at where others are based. From my point of view, Matrix has the advantage of being decentralized (and with the international context that's a good move) and Discord is gaining in popularity.

They both have my vote!

Cheers,
Renaud