I set the following values to empty strings which kept Moodle 2.3 beta Japanese language pack 99% (17906/17908) translated.
parentlanguage,core_langconfig = 'en'
notsupported,tool_assignmentupgrade = ' '
If it's not OK, pelase tell me.
Using AMOS tool for language pack translation
2 strings untranslated in 2.3 beta.
that should work well. Alternatively, you can try the approach suggested by André Yamin at http://lang.moodle.org/mod/forum/discuss.php?d=2231#p2623 (you just have to stage the strings twice and then you can commit them as an empty string).