Document the prefix_default_language parameter to i18n_patterns (#8763)
Document the prefix_default_language parameter to i18n_patterns (#8763)

* Document the prefix_default_language parameter to i18n_patterns
* Update docs/advanced_topics/i18n.md

Co-authored-by: Dan Braghis <2591e5f46f28d303f9dc027d475a5c60d8dea17a@zerolab.org>
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