i just did a test and edited this content with page define editor. it seems that
includes/languages/german/html_includes/define_about_us.php is required for seeing the page in editors dropdown selection.
but the changes affect only the override file at
includes/languages/german/html_includes/MY_TEMPLATE/define_about_us.php.
is this correct?

i assume if i'd remove the override file then the changes will directly affect the file at includes/languages/german/html_includes/define_about_us.php. somehow confusing for the start but i think i understood so far for now.