It is necessary to set the language for each page. Page crawlers obviously recognize the language of the text on the page, but they also require you to add this specific information to your page settings. Besides, if you plan language versions for pages, they will not work well without setting page language correctly.
Set language for home page of the Website.
1. Select the main page of the Website.
2. Go to page settings, "Locatisation" tab
3. Select the language.
From now on, each newly added page will have the default language of the home page.
4. If the page does not have language versions, leave the "Canonical language page" field blank.
Do the same if you create the Website in one language, e.g. German. Language of the page: German, canonical language page: blank.
If you want to create a multilingual website, i.e. you want the english website to have an identical version only in French (or in a different language), read the post about the language versions of the pages.
See also: SEO Tips for begginers