Hi
Yes, I write a constant on a custom field option. Then I make a language override of this constant to the real name in each language (english and spanish) on Administrator and Site. It apears correctly everywhere except on filter options of your plugin.
I think that is related on your layout file filter.dropdown-item.php , when it writes the options variable {options.frontend_value|s}, It seams that it get the constant text after the language override translate the constants. Because if i change your variable for <?php echo JText::_('TEXT_CF_EXPOSITOR'); ?> , it translate it correctly.
I hope I have explained well 🙂