Not touching the Polish files since they appear to be up-to-date (the
scripts also produced changes for these files but they don't seem very
meaningful; I can of course send the resulting files to Roman if he
wants to inspect the changes).
This brings a developer comment to the FlightGear-Qt.xlf files that will
hopefully draw the translators' attention on the fact that they should
put the *up-to-date* address of the "getstart" manual in their language
(use correct address + two occurrences of 'en' to change).
- Take the last FlightGear-Qt.xlf from branch 'release/2020.3'. The
translation is complete from the point of view of Qt Linguist, but the
FlightGear-Qt.xlf files need some refreshing from the C++ and QML
sources.
- Translate remaining strings in FlightGear-nonQt.xlf.
