URL aliases [node:language] repetition issue
That happens because Drupal is using the language prefix to generate the language URLs, and you are using the [node:language] token in the path pattern. As consequence, the language prefix appears twice.
In the first case, as English is the default language, the prefix doesn't appear twice because [node:language] is replaced from an empty string.

Just use [node:title], instead of [node:language]/[node:title] as path pattern.

The reason for allowing the use of [node:language] is that the other method to generate language URLs is through a domain for a specific language. Suppose you set http://exemple.com has domain for the French language; with your path pattern, the URL alias would be something like http://exemple.com/fr/recherche-de-mots, which is perfectly acceptable.