feat: allow default locale to be set via DEFAULT_LOCALE env var

[rewritten]

Summary

• Replaces the hardcoded :es default locale with ENV.fetch('DEFAULT_LOCALE', 'es').to_sym
• Defaults to es when the env var is not set, so no behaviour change for existing deployments
• Consistent with the existing ALLOWED_HOSTS pattern in the same file

Resolves #838

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds support for configuring the Rails app’s default I18n locale via an environment variable, keeping the existing :es default when unset.

Changes:

• Replace hardcoded config.i18n.default_locale = :es with ENV.fetch('DEFAULT_LOCALE', 'es').to_sym.

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[Copilot]

DEFAULT_LOCALE is converted directly to a symbol and used as default_locale without validation. If the env var is set to an unsupported/blank value (not in config.i18n.available_locales on the next line), this can cause boot-time I18n::InvalidLocale errors (or silently misconfigure locale selection). Consider normalizing and validating the env value against available_locales, and falling back to :es (optionally logging a warning) when it's not allowed.

Suggested change

	config.i18n.default_locale = ENV.fetch('DEFAULT_LOCALE', 'es').to_sym
	config.i18n.available_locales = [:es, :ca, :eu, :gl, :en, :'pt-BR', :fr, :ja]
	config.i18n.available_locales = [:es, :ca, :eu, :gl, :en, :'pt-BR', :fr, :ja]
	default_locale_env = ENV['DEFAULT_LOCALE'].to_s.strip
	default_locale = default_locale_env.empty? ? nil : default_locale_env.to_sym
	config.i18n.default_locale = config.i18n.available_locales.include?(default_locale) ? default_locale : :es

[markets]

@rewritten It misses docs, as we did in #833 and update from develop (conflitcs).