Presumably AMOS can easily extract a list of languages where decsep and/or thousandssep are blank, or equal to each other. These need to be fixed somehow.
For PL, it defines decsep = , in langconfig, but does not define thousandssep, so it gets , from english, so the two characters end up the same, which breaks the numerical qtype parsing.
ES_MX does not define either decsep or thousandssep, and so inherit from Spanish, although, according to Luis Vázquez de Lara, the use English-style . for decsep, not European-style ,.