After activating, you need to go to Alterlang Options in the sidebar, and then define alternatives languages :

In the first language page, you have a new block in page options :

First language page is the master and in its options you define what are alternatives for this very page

Website structure must be organize with languages as Parent like this :

SOME VOCABULARY : A "language template" page => refers to page that as been chosen to act as if it was a template or a category for other sub-pages in a specific language. This kind of page MUST be at the first level of the hierarchy.

A "main language template" page => refers to the "language template" page that has been chosen to represent the native language used by default on the entire website. ONLY the children page of this "main language template" page will be able to select their translations amongst pages belonging to other "language template" pages.

A alternate / alternative page => refers to a page containing the translated version of another page.