The language packs that have (partially or completely) translated language strings that include the qtype_stack plugin are:
Finland Swedish (sv_fi)
Spanish (International) (es)
Spanish - Mexico (es_mx)
The mexican Spanish strings are 100% translated, but the translator (me) is a veterinarian who hasn't studied maths since high school in1973. I fear that some of these translations are quite literall and might need some changes.
Any help with suggestions for improving the qtype_stack language strings for the above language packs will be very appreciated. Translation instructions for all Moodle language strings are at https://docs.moodle.org/dev/Contributing_a_translation .
Mathematicians wishing to contribute a different language pack translation for qtype_stack wil also be greatly appreciated.
Thanks in advance.