As I said here German translation
The launchapd is the place to do the translations (either online or by downloading the .mo files), although I can see that in some cases the context might not be quite clear.
The referenced php files etc could help you identify where the line is used, as the name of the file should point you to right direction, but again I know it might not always be clear if you’re not overly familiar with Xibo.
The line that you’ve mentioned is in Dayparting page -> Add/Edit daypart -> Exceptions tab
I’m afraid I don’t think launchpad can be more user friendly.