The translation install should be activable by option or by an utility function after xmlschema import.
This because someone might not want to turn on translation or might want to use another custom translation.

Yes, I think about some settings option or environment variable. But all strings in code must be wrapped in _() function, which initializing in translation.install(). And I don't know how make translation optionable. Maybe language code should be in settings.

This is a library and currently it doesn't have settings, and I would prefer to avoid this in order to keep it simple.

As i saw installed translations are stacked into gettext module internals, so i think that a reasonable manner to handle this is to provide an utility to add package translations, also parametrized with language codes. I don't know how currently, this could be decided after looking at solutions used in other packages.

Add initialization via schema = xmlschema.XMLSchema11(xsd, use_translation=True)

I will add copyright info on this part.
If it's not a problem also add author info.

Of course, it was autogenerated by xgettext utility

A script or an utilily function for regenerate this POT file could be useful.

Maybe need adding Localization section in README with helpful utilities. In fact they may be not easy to use.

# generate *.pot file with all strings, that must be translated
xgettext xmlschema/validators/simple_types.py xmlschema/validators/exceptions.py -o xmlschema/locale/xmlschema.pot

# update locale *.po file from *.pot
msgmerge xmlschema/locale/ru/LC_MESSAGES/xmlschema.po xmlschema/locale/xmlschema.pot -o xmlschema/locale/ru/LC_MESSAGES/xmlschema.po

# generate *.mo file from locale *.po
msgfmt xmlschema/locale/ru/LC_MESSAGES/xmlschema.po -o xmlschema/locale/ru/LC_MESSAGES/xmlschema.mo

Good to use locale/__init__.py for this, but instead of providing extra arguments to schema class it could be better to provide an helper function like this:

import gettext
from pathlib import Path

translation = None


def install_translation(localedir=None, languages=None, class_=None, fallback=False):
    global translation

    if localedir is None:
       localedir = pathlib.Path(__file__).parent..joinpath('../locale').resolve()

    translation = gettext.translation(
        domain= 'xmlschema',
        localedir=localedir,
        class_=class_,
        fallback=fallback,
    )
    translation.install()

importing install_translation into package __init__.py the function can be used to activate xmlschema provided translations after package import:

import xmlschema
xmlschema.install_translation()
schema = xmlschema.XMLSchema(...)

or to install other custom translations for the xmlschema domain providing optional arguments:

import xmlschema
xmlschema.install_translation(localedir='../mylocale', languages=['fur'])
schema = xmlschema.XMLSchema(...)

If you need to keep the translation setting at each schema init you can collapse to a single extra argument, eg.:

use_translation: Union[None, Path, str] = None)