i18n and L10n in TYPO3 Flow allow software to be localized for different languages and regions. Key aspects include translating text, formatting dates and numbers based on locale, and localizing other resources like images. Message catalogs in XLIFF format are used to store translated text mapped to source text identifiers. The f:translate ViewHelper and Translator API provide access to translations, while formatters handle locale-specific rendering. Support for plural forms and placeholders in translations is also covered.