In section editing dialog the text for the course-format specific section name identifier is not displayed anyway in custom course formats. In these cases the language file "lang/.../moodle.php" is used because it is hard-coded in "course/format/editsection.php" (line 55 in my moodle 1.9 dev version from latest nightly build). But this global language file does only support the build-in course formats and is the wrong one in these cases. For build-in course formats like weekly, social, etc. all works fine but it cannot be changed to specific language files in custom course formats in spite what the course format extension specification says. It is globally hard-coded, but it should not be
To get the course formats being modular and able to be extended it is necessary to change the call of get_string() in editsection.php to use the language file of the actual course format instead of the global language file if such one is available. Maybe it will be a good solution to define the language constants for format and section names for all (also build-in) course format in its own language files?
Unfortunately I do not know which dependencies of this problem are affected, so I cannot change the code myself.