Shibboleth IDP 3 uses Spring which makes internationalization (i18n) very easy.
MessagesTranslation describes how to add your own or crowd sourced translations.
Switching locale on the login page has instructions on how to add a link on your login page to switch languages.
Human Readable Attribute Names has instructions on how to add translations of attribute names to the AttributeResolver configuration, e.g. to display these names during attribute release consent.