The language pack for a plugin doesn't need separate downloading. If it is translated in your language, then it is in the language pack of your language.
Assuming the language you are searching: there are 66 strings out of 804 missing for that plugin (including overview, regrade, .. etc)
Have you updated your language packs?
ah, you mean the files in offlinequiz/tutorial/en/
They will never be in the language pack, because they are in the wrong place to be picked up by AMOS.
There is a bug filed for it on https://github.com/academic-moodle-cooperation/moodle-mod_offlinequiz/issues/63