The menu shows
xterm -u8 -fa Mono -fs 9
and the comment about starting it from the command-line says
xterm -u8
Given that the question was asked in 2015, that would be no more recent than patch #320. Support for TrueType font sets (fallback fonts) was introduced in 2018, patch #338.
There are a few possibilities:
most of the TrueType fonts are not unicode fonts, but rather ISO-8859-1 with a handful of characters outside that set. You can see the coverage using
xfd -fa Mono
without the -fa
option, xterm defaults to bitmap-fonts, which may come with reasonably good Unicode coverage (depending on what you want).
your system locale may not use UTF-8 encoding; if your shell sets the locale in its initialization script, you can see differences
likewise, if your shell sets different locations for the resource files than the system, that can make a difference.
But likely the difference lies in the font choice. For instance, on my OSX machine, "Mono" is shown in xfd
as Bitstream Vera Sans Mono-12:style=Roman, which (see Font Squirrel) has that sort of limited coverage.
There are TrueType fonts with good coverage (better, anyway). Since that version of xterm did not implement font sets, it shows only one font. Here are a few interesting links on the topic of TrueType versus Unicode: