Hello. I tested it in group:
1) navigate to group
2) upload one photo to timeline
3) add text
4) post stream item (one photo with text)
5) wait for few comments
Result: language display notification like one user commented this photo (no PLURAL).

I suppose it one (or few) of this lines:

APP_GROUP_PHOTOS_COMMENTS_YOUR_PHOTO="%1$s commented on your photo"
APP_GROUP_PHOTOS_NOTIFICATIONS_COMMENTED_ON_YOUR_PHOTO="%1$s posted a comment on your photo"
APP_GROUP_PHOTOS_NOTIFICATIONS_COMMENTED_YOUR_PHOTO_SHARED_ON_THE_STREAM="%1$s posted a comment on the photos you shared"
APP_GROUP_STORY_USER_COMMENTED_ON_YOUR_SHARED_PHOTO="%1$s posted a comment on the photo you shared in {b}%2$s{/b}"
APP_PHOTOS_COMMENTS_YOUR_PHOTO="%1$s commented on your photo"
APP_PHOTOS_COMMENTS_YOUR_PHOTO_CONTENT="{b}%1$s{/b} commented on your photo: %2$s"
APP_USER_PHOTOS_NOTIFICATIONS_COMMENTED_ON_YOUR_PHOTO="%1$s commented on your photo"
APP_USER_PHOTOS_NOTIFICATIONS_COMMENTED_YOUR_PHOTO_SHARED_ON_THE_STREAM="%1$s posted a comment on the photos you shared"

Thanks.