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).
This translated string has been removed from Translations/en/menu.xml in
commit ff87cf4257. Remove its translations
from the other Translations/*/menu.xml files that still have it.
Also remove <intl include="Translations/locale.xml"/> from
preferences.xml. Something equivalent based on readProperties() is done
in FlightGear (cf. FGLocale::FGLocale() in
flightgear/src/Main/locale.cxx).
Run script to identify missing/obsolete items:
de/options.xml is missing 192 translation(s).
de/menu.xml is missing 20 translation(s).
es/options.xml is missing 12 translation(s).
es/menu.xml is missing 78 translation(s).
fr/options.xml is missing 0 translation(s).
fr/menu.xml is missing 0 translation(s).
it/options.xml is missing 212 translation(s).
it/menu.xml is missing 78 translation(s).
nl/options.xml is missing 54 translation(s).
nl/menu.xml is missing 1 translation(s).
pl/options.xml is missing 81 translation(s).
pl/menu.xml is missing 76 translation(s).
pt/options.xml is missing 212 translation(s).
pt/menu.xml is missing 86 translation(s).