Just tested your site, I see nothing wrong. There was a bug reported lately regarding "age age" showing up.
There might be a bit of a misunderstanding here. You do not (and can not) set the language of webtrees since it is multilingual. It roughly works as follows.
Suppose a visitor opens a page on your webtrees site. The browser sends a list of languages the user understands to the server. The server will go from top to bottom through the list until it finds a language it supports, and serves the page in that language. Not many people know it, but you can configure your web browser and set Dutch as your primary preferred language, and set English as second option. Once you have done that, webtrees will always show in Dutch to you.
Now suppose the visitor changes language. This is stored in the session information of webtrees. As long as the session lives, the selected language will be used.
Now suppose you have an account and log on. Only now webtrees knows who you are. It will look up your user profile in which a preferred language is stored, and it will use that language. If you change the language in the menu, it might be changed in the user profile, or not (have to test that)