Extrafields translations

To create extra fields untranslatable I've always done the following:
create my file, template_name/etc/extrafields/extrafieldfile.xml

This Oportunity needed in 2 languages. so wrote strings of translation, but translate them into en-GB.tpl_template_name.ini. the translation is not working.

But the translation if it works when I created the file, template_name/etc/FORM/com_content.artcle.xml. even though it has only the following code:

The question is; this is my error? T3 error ?; or this necessarily work well?