And you can also do this directly in your Thymeleaf view like so:
<span data-th-text="${portal.localize({'_key=menu'})}"></span>
This will check your current site locale and find the “menu” keyword in the correct phrases-file.
Make sure your locale matches the settings in admin. “en” is the shorter English locale, while “en_us” is for American English. However Enonic will “waterfall” down to find the correct phrases file and always use the default one as an last resort. If no match is found anywhere, “NOT TRANSLATED” (or similar) will be printed.