The IdP includes some, but by no means exhaustive, internationalization (I18N) support:
MessagesTranslation describes how to add your own or crowd sourced translations.
Human Readable Attribute Names has instructions on how to add translations of attribute names via the AttributeRegistryConfiguration or AttributeResolverConfiguration, e.g., to display these names during attribute release consent.
There is also a contributed HowTo for Switching locale on the login page via a link on your login page to switch languages. This is not standards-compliant and is not generally an advisable approach, or one that we test or can guarantee will work consistently.
Note that in the absence of explicit guidance from the browser, the OS default locale will usually dictate which messages are chosen.