2302 Storefront breaks on language switch

Hi!
I have been trying to add a second language to a 2302 store.
Have made admin language settings accordingly so code, priority etc in Localisation is set, as well as Default language in System > Local.

On frontend the switcher shows up correct with flags but as soon as I switch language the site breaks with a "decoding error".
If I click backwards a couple of times I get to a previously loaded page where I'm able to switch the language back - and the site works again.
Admin still works.

Cleared vqcache and system cache between tests/changes.

Has this got anything to do with browser language auto detection?

From what I remember, it may be the file encoding of the newly added language files.

You have to check through the files and ensure that the language files are saved in UTF8 without BOM.

Thanks!
Checked the encoding and it seems to be correct UTF-8 without BOM.

What is the full decoding error message you are getting?