I just figured out https://github.com/pavouk/lgi/issues/173.

This is lgi, the Lua GObject-introspection bindings, failing to handle Regress correctly. The following XML

    <constant name="DOUBLE_CONSTANT"
              value="44.220000"
              c:type="REGRESS_DOUBLE_CONSTANT">
      <type name="gdouble" c:type="gdouble"/>
    </constant>

is turned by g-ir-compiler into a constant with value 44. This happens with LC_ALL=de_DE, but does not happen with LC_ALL=C, so is likely due to the different decimal separator in Germany.

With some more digging, I found out that the code since always used strtod() to parse these constants (function parse_float_value() in girepository/girnode.c). This does not explain why things suddenly broke...

However, recently-ish all main() functions where changed to call setlocale(): https://github.com/GNOME/gobject-introspection/commit/74e4542739df792d4a1f42eecd756506140703b0#diff-03258a1fcced22a7eb876058d39e9118
Thus, after this commit, strtod() expects 44,22 instead off 44.22 and aborts at the unexpected ".".

There might be other, similar locale-related problems in this code, I don't know.