Your default language is where original catalog and storefront content is written. Additional enabled languages can hold translated versions of that content. The number of enabled languages depends on your plan.
Enabling a language prepares localized search documents and makes the language available for assignment to storefronts. Customers only see it on storefronts where it is served.
Before switching to a plan with a lower language allowance, reduce the enabled set to that plan's limit. The plan change is blocked while too many languages remain enabled.