turn missing translations into a warning (#3752)
turn missing translations into a warning (#3752)

This is not fatal and can be mistaken by new users for the cause of all their problems, like I did in #2621.
Markismus/koreader,mihailim/koreader,Hzj-jie/koreader,pazos/koreader,poire-z/koreader,Frenzie/koreader,NiLuJe/koreader,koreader/koreader,koreader/koreader,lgeek/koreader,Frenzie/koreader,mwoz123/koreader,poire-z/koreader,NiLuJe/koreader,houqp/koreader