Adding text in more than one language (at the same time)

WordPress doesn’t have built-in features for a site that is served in multiple languages. Yes WordPress has features for translating the UI labels into different languages, but not for the actual content. You are going to need to use a plugin that basically keeps multiple versions of each Post for each language and has means for the user to select the language. The WordPress Codex has an article that overviews the different ways to achieve a multilingual setups.

Articles to help you pick a plugin