https://docs.moodle.org/dev/String_API
Shortly, you can't simply add a new file into lang/en/ and expect it start working. The new strings are supposed to be part of a plugin.
If you are customizing Moodle for some local needs then a plugin type "local" might be suitable for you. That was introduced for these purposes. Usually, a proper plugin type such as admin tool or block may suite more. It really depends on the nature of your customizations.