Dear Long,
I can't still figure out what could be the problem, but I will take a look on Loco translate. More info:
The MBCPT plugin itself is loading in English (not a big problem for me), but what I refer too is the content of each label. They are also in English, of course, but I can't modify them permanently. Steps:
- I do create a "Foo" custom post, all OK.
- I name plural and singular, "Foos" anf "Foo", all OK.

- I go to CPT labels and change from, for example, "Add new Foo" to "Adicionar Novo Foo". And I change "All Foos" to "Todos os Foos"... And it is ALMOST OK.

The changes are made, the labels are indeed modified, but the text field itself, on the plugin, retains the old value in English. If I need, for some reason, edit the CPT again, I need to rename aaaall labels once more, otherwise the will be re-saved with the original English values the plugin provides.

If I radically change it to any crazy text, it get's saved, but upon entering the CPT's edit screen again, the value will not be there.

But is is strange, because the same problem happened using just free the MB and MBCPT on on a different theme, a commercial one (mine is all custom and I use premium AIO). On both themes I just activate MB's plugins and use a custom code to "boot" the metaboxes (did learn on your old tutorials). Perhaps my custom way of starting MB internals is breaking something?
This is the way I do:
function teialocal_register_meta_boxes() {
include 'boxes/post.php';
include 'boxes/page.php';
if (post_type_exists('case')) { include 'boxes/case.php'; }
if (post_type_exists('event')) { include 'boxes/event.php'; }
return $meta_boxes;
}
add_filter( 'rwmb_meta_boxes', 'teialocal_register_meta_boxes' );
I will try MB + MBCPT on a default WordPress template to check if it happens there as well.
Thanks!
G.