Update them from the latest contents of the
$FG_ROOT/Translations/default/*.xml files (default translation).
This was done with:
fg-update-translation-files \
--transl-dir="/path/to/fgdata/Translations" \
merge-new-master de en_US es fr it nl pl pt zh_CN
These files have been generated from the existing XML files (cf.
fg-convert-translation-files in fgmeta).
Translations/en_US/FlightGear-nonQt.xlf is for a proper English
translation, where for instance "found %n airport(s)" would have two
plural forms, "found %n airport" and "found %n airports" (most
non-plural strings can be taken verbatim from the default translation,
and at this point there is no plural form at all yet).
