Thanks for pointing me in the right direction
Turned out
I18n.translations[currentLocale].js.underline_text = settings.underline_text;
should be
I18n.translations[currentLocale].js.composer.underline_text = settings.underline_text;
Thanks for pointing me in the right direction
Turned out
I18n.translations[currentLocale].js.underline_text = settings.underline_text;
should be
I18n.translations[currentLocale].js.composer.underline_text = settings.underline_text;