I've just fixed this in the upstream TQSL 2.3 beta repository, along with another unrelated issue when compiling on Stretch.
This isn't a Debian-specific issue - it's a defect that will happen on any system without the English localization files installed.
*** a/apps/tqsl.cpp 2016-07-31 10:50:07.000000000 -0400
--- b/apps/tqsl.cpp 2016-07-31 10:49:34.000000000 -0400
***************
*** 4736,4742 ****
lang = (wxLanguage) lng;
if (lang == wxLANGUAGE_UNKNOWN) {
! lang = wxLANGUAGE_ENGLISH;
}
for (lng = 0; (unsigned) lng < sizeof (langNames); lng++) {
--- 4736,4742 ----
lang = (wxLanguage) lng;
if (lang == wxLANGUAGE_UNKNOWN) {
! lang = wxLANGUAGE_DEFAULT;
}
for (lng = 0; (unsigned) lng < sizeof (langNames); lng++) {