Describe the bug

Organizations with hyphenated locales (e.g. "es-MX") are not working properly when saving forms.

To Reproduce

Some steps to reproduce the behavior:

1. A clean instalation of Decidim, with only one organization with one hyphenated locale as the only locale (es-MX).
2. Login as Admin user
3. Go to AdConfiguración / Editar organización
4. Click Actualizar at the bottom of the page

Expected behavior
It should save the form correctly.

Screenshots

Extra data (please complete the following information):

• Device: Desktop
• Device OS: MacOs
• Browser: Firefox
• Decidim Version: 0.25.2

Additional context

I think the problem is related to file https://github.com/decidim/decidim/blob/develop/decidim-core/app/forms/translatable_presence_validator.rb

line 12: translated_attr = "#{attribute}_#{default_locale_for(record)}"

Attribute "admin_terms_of_use_body" with hyphenated locale "es-MX" becomes "admin_terms_of_use_body_es-MX", but it should be "admin_terms_of_use_body_es__MX".

Also, for reference, in a form like in file https://github.com/decidim/decidim/blob/develop/decidim-admin/app/forms/decidim/admin/organization_form.rb

In line 36: translatable_attribute :admin_terms_of_use_body, String gets injected as admin_terms_of_use_body_es__MX when locale is "es-MX".