Better translation support in all languages.

• Adapt keywords to display the presentation article of a section in the all languages
• Adapt skel to display the presentation article of the home page in the all languages (scholar.exoplanet.eu/)
• Adapt keywords to display the presentation article of the home page in the all languages
• Install the plugin to get the language set up for the browser
• Add an alert at the top of the website to explain the change and instruct users to delete cookies if necessary. Delete this after one or two weeks.