Hi, welcome to the forum!
I’m not very technical, but basically Slack is a workspace host site on the web. There is a group called Welsh Speaking Practice which uses it and this offers an opportunity for anyone anywhere at any level to practice speaking Welsh. You can chat 1-to-1 with other learners or more fluent speakers or join group conversations and you can do it just with audio or with video. It’s a very friendly group and a really good way to improve your speaking and listening skills.
There is more info in this thread:
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