I've checked now the lang files in standard Moodle package and made a first analysis for translation priorities.
A standard language package includes nearly 400 language files and more than 17.500 strings today. Most of the files are very small.
The files includes often strings that are used for admins, for teachers and for students. Butthere are also files with strings that are only visible for teachers and admins or only for admins.
Most translators wants to concentrate first on language elements that are visible for students. The attached EXCEL file includes a list of all language files, the numberof strings and a short descriptions that should help to understand what this file is about. In the first column you'll find a categorization with priorities for translation.
Priority A means should be translated. This category includes files with strings which are visible for students. This doesn't mean all strings are visible for students.
Priority B means strings are used for teacher views. In some cases this also means the strings are from an activity or module that is not used as often as the modules in category A.
Priority C includes admin only language elements and seldom used modulesor modules that are not very often enabled.
The priorization is based on my personal experience. I'm more than happy to get feedback an suggestions to optimize by other users.
Nearly 50% of the strings and files are for admin only or categorized with low priority C. One quarter of the strings are categorized in B and A. Than means ~4.500 strings could be visible for students.
The files 'core' (1.600 strings) and core_admin (990 strings) are priorized with 'A'. Both files include two third or more of strings that are invisble for students. This are the largest lang files in Moodle system. But we have no simple scheme to find the 'admin only' strings in the file.
My personal calculation is that ~ 3.000 strings are visible for students only.