Not sure what the answer is you're looking for but the information is stored in the table 'sys_categories' and the table does not contain a language key.
If you would like to have categories as part of the language file, it means modifications to the table and to the script.
Simply explained;
/m/photos/browse/category/Fun
This url means that we want to browse photos in the category fun.
the photo module script uses the category from the URI to determine the category to show. It than uses that category name to do a search for everything within that category
If you would use language keys, it would mean the script has to be modified in order to "translate" that category to the correct language key before it can do a search. Not only would you have to do this for photo's, but also for events, video's, etc.
When adding a new photo it will show you the list of all categories with type 'bx_photos'
(SELECT 'category' FROM 'sys_categories' WHERE 'type' = "bx_photos')
Again, if you want to add a translation, it would mean extra fucntionality in the scripts to get the correct language and the ID's to get the correct categories from the 'sys_categories' table with type 'bx_photos'.
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