Rapidweaver by default sets the Site language to English. Running many tests on sites I run and other sites the tests all had a commonality, they made reference to the fact that there was no language declaration. This was curious and this is what I think is happening.
lang=“en” is not being written by Rapidweaver - to get it to work you have to select another language and then change it back to English and then it works.
Maybe Rapidweaver can confirm?