A group's language list decides which languages members can read the site in, and therefore how many times every event description has to be written. Adding a language is cheap; keeping it filled is not.
Languages in the group settings lists the languages currently active, marks one as the default and offers a dropdown to add another. The page argues for restraint on its own: every language you enable needs all your content translated by hand, and a missing translation shows up as incomplete content rather than as a fallback.
Decide whether you will actually translate
The page says it plainly: only add languages your group really uses or holds its meetings in. An empty language is worse than a missing one.
Add it
Pick it from the dropdown and press Add. It appears in the active list immediately.
Fill in the existing content
Every event and CMS page now has empty fields for the new language. Nothing translates itself; go back through what already exists.
Removing a language leaves content behind
Orphaned entries stay in the database, invisible and uneditable - and recurring events keep copying them forward into new occurrences. Cleanup is where you get rid of them, and it deletes permanently with no undo. Content in the active languages is not touched.