Hi James,
thank you so much for your help, this solved it! 🙂
I had to add 2 filters and replace every _x() function with __().
The filter:
function custom_acf_settings_localization($localization){
return true;
}
add_filter('acf/settings/l10n', 'custom_acf_settings_localization');
function custom_acf_settings_textdomain($domain){
return 'my_theme';
}
add_filter('acf/settings/l10n_textdomain', 'custom_acf_settings_textdomain');
And replace
'title' => _x('Related Post', 'ACF Related Post', 'my_theme'),
with
'title' => __('Related Post', 'my_theme'),
Now it’s working perfectly! Thanks! 🙂