On my french keyboard xcalc only accepts input from the numeric pad.
Digits keyed from the main keyboard are ignored. Gnomecalc doesn't have
this limitation.

This is because the key mappings in the translation list in app-defaults
only list the main set of number keys with a modifier of "None", so they
don't work when NumLock is active.  We can't remove that modifier since
then holding down shift and hitting one of them would enter the number
instead of doing the intended operation (!, *, (, or ) for instance).

Instead I've submitted a merge request upstream to add translations for
those keys with Mod2 (aka NumLock) in the modifier list - you should be
able to edit the XCalc app-defaults file on your system to do the same
until a new release is published:
https://gitlab.freedesktop.org/xorg/app/xcalc/-/merge_requests/11

This should be fixed now in the new upstream xcalc-1.1.2 release.