OK, I figured that out, it was my mistake. The issue was from a custom field type, for which I wrote some javascript, like documented in Adding custom javascript to fields.
I introduce a dependency to acf-field-group.js, which forced loading it on every pages (and therefore activated the unwanted validation which was using unloaded internationalized string).
I put my script file back on footer, removed the dependency, and it all works fine.
The reason I first added this dependency (which is not documented), is that I had JS errors “acf is not defined” in pages where acf is not present (as wp_enqueue_script adds it to every pages). Before using acf.add_action, I added a test to ensure it is pertinent to go on :
if (window.acf) {
It seems that adding the dependency to acf-input.js do not cause issues, and may be a proper solution (to ensure WP loads custom scripts after it).
Thanks a lot for your support.
I’m not using any translation plugin. Maybe I should have used the word internationalization instead.
The title validation issue happens on all option pages created using acf_add_options_page.
I also deactivated all other plugins, which do not solve anything.
The ‘title_is_required’ label translated to nothing issue is present regardless of the selected language of WP backend.
