These are forums dedicated to the pastime of creating artificial languages and related topics, such as linguistics and worldbuilding.

The CBB Forum: https://cbbforum.com/

The Zompist Bulletin Board (ZBB): https://www.verduria.org/

Both use phpBB and there is a fair amount of overlap in userbase. The CBB seems to be more active, and is IMO the more pleasant community as the ZBB allows IRL political discussion which is (tacitly if not explicitly) discouraged on the CBB.

Both forums deal with the same topics, primarily constructed languages (both ones you’re making yourself and ones featured in media like Klingon (Star Trek), Na’vi (Avatar), Dothraki (Game of Thrones), and of course the many, many languages of Lord of the Rings.

Other topics of discussion include constructed worlds (conworlds), which may or may not feature constructed languages (conlangs); linguistics, and other arts and sciences related to worldbuilding.