https://discourse.gnome.org/t/tu-usted-in-spanish-translations/748 discusses how GNOME, the desktop environment of standard Ubuntu, only has one version of Spanish.
This may be different with the other 'flavours' of Ubuntu, listed here, because they use different desktops other than GNOME. Linux does allow for dialectical variations of Spanish (not surprising, as there are more dialects of Spanish than any other major language).
You may wish to consider loading other flavours onto a LiveUSB, and trying them to see which has superior support for Mexican Spanish.