Manage Languages opens the full language catalog with your enabled set checked. Check or uncheck languages and save — the selection replaces the whole set at once. The default language is always included and can't be unchecked, and the picker stops accepting new languages at your plan's limit.
What blocks a removal
A language that's assigned to a storefront can't be removed while that assignment exists — remove it from those storefronts first, then from the store. The default language can never be removed directly; change the default first.
Translations data
Each non-default language accumulates translations. Remove Translations Data permanently deletes a language's translations and translation jobs while leaving your source content untouched — useful for starting a language's translations over.