It's a frequent problem in almost all languages except English, I guess.
[ alt ] + [ tab ] and [ alt ] + [ key above tab ] shortcuts are often associated with « application switcher » and « window from an app' switcher ».
The problem here is the symbol on [ key above tab ] depends on language and keyboard mapping.
⋅ on most english / qwerty keyboards [ key above tab ] is [ ` ] ( grave )
⋅ on a french / azerty, it's [ ² ] and this will vary with language and mapping.
Any app' using [ alt ] + [ key above tab ] shortcut should not rely on the symbol on the key but only on position of that key after keyboard mapping, sort of.
Therefore if the shortcut calls for [ Alt ] + [ grave ] instead of [ Alt ] + [ key above tab ] in many languages you end up with some awkward gesture, depending on [ grave ] position on your keyboard mapping ( which is [ 7è ] key above [ y ] on a french azerty, quite far from [ tab ] ).
It's not only Mate related, it's unfortunately true in all DE.