Once you get past this initial hurdle of figuring out how to set the locale, the locale itself does not always apply fully to the web client's strings. So sometimes I found that if I set the locale using the cookie, it'll change the menu names, but not the strings on the links or page content. Or it'll change the page content, but not the menus. It takes a couple browser refreshes to fully translate the page. So perhaps some caching issue during i18n switch?
Some additional testing notes...
Once you get past this initial hurdle of figuring out how to set the locale, the locale itself does not always apply fully to the web client's strings. So sometimes I found that if I set the locale using the cookie, it'll change the menu names, but not the strings on the links or page content. Or it'll change the page content, but not the menus. It takes a couple browser refreshes to fully translate the page. So perhaps some caching issue during i18n switch?