Hi @pictibe, I have looked at rosa. Unfortunately they indeed forgot to pass those strings through translator function __() and the text will never be translated even if we implement responses to language switch buttons for those custom header fields.
To make it work you may edit three lines in /wp-content/themes/rosa/templates/page/header.php file:
Replace line
$subtitle = trim( get_post_meta( wpgrade::lang_page_id( get_the_ID() ), wpgrade::prefix() . 'page_cover_subtitle', true ) );
with
$subtitle = trim( __(get_post_meta( wpgrade::lang_page_id( get_the_ID() ), wpgrade::prefix() . 'page_cover_subtitle', true ) ));
Then replace line
$title = get_post_meta( wpgrade::lang_page_id( get_the_ID() ), wpgrade::prefix() . 'page_cover_title', true );
with
$title = __(get_post_meta( wpgrade::lang_page_id( get_the_ID() ), wpgrade::prefix() . 'page_cover_title', true ));
Then replace line
$description = get_post_meta( wpgrade::lang_page_id( get_the_ID() ), wpgrade::prefix() . 'page_cover_description', true )
with
$description = __(get_post_meta( wpgrade::lang_page_id( get_the_ID() ), wpgrade::prefix() . 'page_cover_description', true ));
Then using [:en] or <–:en–> in Subtitle, Title and Description fields starts working.
Unfortunately, you would have to do those edits on each their update, or you may contact them to ask to make this change permanently. It is a valid request, it will not hurt any other clients. They should be happy to put such a fix in.
Fortunately, they implemented rosa in a child-theme-friendly manner. So, you could create a child theme and override that file in child theme. This way your changes will survive the most of updates painlessly.
When I played with it, I actually did the child theme and it worked, I will send you zip file with child theme via e-mail. It has two files only. You would need to unpack it under /wp-content/themes/ and activate “Rosa Child” theme in WP. You will also need to re-enter under Rosa-Child a few customizations, like menu allocation, etc, which should not be a lot of work.
Please, let me know if this would work for you.
Thanks a lot for your feedback.