USER: scenario: precise it is run with a browser with English default language

For any scenario, default behaviour is to audit the page with English language.

Consequences:

• if the application to be audited is multi-language, you must include in your scenario the actions required to explicitly change the language
• an Asqatasun administrator may install a localized Firefox ESR (for instance French Firefox for an Asqatasun instance in France)

Both those info should be clearly written in the scenario documention