Fallback to hsb
According to some study, hsb and dsb are 86 % mutually intelligible in written form. Instead of showing a large part of the interface in English, when a translation is missing in one language it would be nice to show the translation to the other (language fallback). Do you translators agree?
Yes, definitely. Speakers of Lower Sorbian should generally understand Upper Sorbian way better than English.
I am not a native. I have never lived in Lusatia. However, I think this is a very good idea. A native speaker of one language probably should have some contact with the other language, as there is one common organisation representing interests of Sorbs as a minority in Germany. I cannot remember any case of a translation being offered when in a radio/TV programme a speaker of the 'other' language was interviewed.