I18N Multilevel navigation & breadcrumbs symbol "_" in URL

[mvlcek]

So don't use "_" in page slugs.

I can not use the symbol "_", as this symbol is used in the URL of the old pages on the site, which stand at the other CMS
on the site does not plan to use other languages, such as whether it is possible to remove / change this separator '_'

No, it is not possible to change this separator.
But maybe you can create redirect rules in your Apache .htaccess file to redirect e.g. "/my_url" to "/my-url".