Then you must checked "Show language" in the articles type panel.
Then when you edit an article in a type that has the language checked, you will see the languages drop down list in the edit article panel.
If you want the modules related to articles like last_articles to display only the articles that are in the user language, you need to create as many article modules as languages and to set for each the param lang= in the module configuration. (ex lang=fr or lang=en)
If you want your Article Home Page to show up only with articles in the user language, depending on your need to can do 2 things:
1) each article has a translation. In this case, be sure to set each article in a set of translations.
The Article Home Page will then display only the articles of the user language.
Note: If an article is not in a set of translation that has an article for the user language, this article will appear.
2) you have different articles in different languages not realted to each other. In this case you must use the url tiki-view_articles.php?lang=en if you want to select only English articles (tiki-view_articles.php?lang=fr to select only French articles). The url must be set in your menu or in each link to the Article Home Page