Moodle - new (not customized) translations

I created some additional webpages (php files) in Moodle. They're accessible of course from the appropriate path (e.g. "..my_folder/page.php"). Now I'm trying to find a solution for adding new translation entries. I noticed that I can edit existing translations in e.g. "[moodledata]/lang/[language-code]/[filename].php" files, however adding new files, or even adding new string to the existing files, has no any effect. After clearing a cache they still seem to be "invisible" for Moodle. I don't know how it works, it must be quite complicated? **How can I add new (not modify old) translation strings for Moodle?** Thank you.