I had a customer who wanted to specify a different site language for their events page made with Matukio. His main language was German and he had a single page, which had to be in English. I've wasted more hours than I'm comfortable to admit on this issue and that is why I thought that I'll share the solution with you, in case you need it.
If we were dealing with a standard multilingual site, the solution to this issue would be pretty straight forward. Define a new content language, enable the language filter plugin, create a menu item for our page and set the language to English. If we go this way we run into the problem that once a user lands on the English version of the page, all links on the site will get /en appended to them. But we don't want this. We want that the links stay as they are.