[fix, lang] More accurate auto-DPI text (#4537)
[fix, lang] More accurate auto-DPI text (#4537)

Simply don't mention any values when the device DPI isn't known.
A more robust auto-DPI info function could be added to Screen
but I'm not sure if it's worth the trouble.

Fixes https://github.com/koreader/koreader/pull/4389#issuecomment-454552446
koreader/koreader,Frenzie/koreader,NiLuJe/koreader,Hzj-jie/koreader,NiLuJe/koreader,pazos/koreader,Markismus/koreader,poire-z/koreader,koreader/koreader,houqp/koreader,mwoz123/koreader,Frenzie/koreader,mihailim/koreader,poire-z/koreader