I read in the documentation that the pages related to the OOTB subscription center are automatically translated into 21 languages. I would like to know if there is a way to manage the languages that are not included in the list, or the pages will be displayed in English in any case for those scenarios.
Unfortunately the OOTB Preference/Subscription center is pretty fixed. This is one reason, among many, that many people develop custom preference centers. The primary reason, of course, being that the current OOTB center is design poor.
There's lots of examples on how to build a custom preference center utilizing CloudPages and then entering a support ticket to replace %%profile_center_url%% with your new CloudPage path. Keeping it within the SFMC environment allows you automatically send over subscriber info such as _subscriberkey, email, and for your use case, locale (if know), and then dynamically control the content on the page to support whichever locales you require.