Document default language configuration

[mintlify]

Added documentation explaining how to set a default language for multi-language sites using the 'default' property in the languages array. This addresses a common question about configuring non-English default languages in internationalized documentation.

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Note

Low Risk
Documentation-only changes with no runtime or configuration parsing changes.

Overview
Updates the internationalization guide to document how the default language is chosen for navigation.languages, including examples for reordering the array or using a default: true flag.

Adds guidance for single-language sites by documenting a docs.json navigation setup that omits languages to avoid showing the language switcher.

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