The "isMultilingualSitemap()" method returns false in some cases even when there are enabled language URL negotiation methods.
Install Domain Country Path module, enable "Country Path Language Handler URL" plugion for content language detection.
Add two or more site languages.
Translate some content.
Generate sitemaps. Alternate URL column should be empty where it is expected to contain HREFLANG alternate links to translated content URLs.
Based on my review, I opened a ticket with the Domain Country Path module, linked here as a related issue. I'm not sure if the problem lies with that module, or with core's management of the language types config, but that issue hopefully will useful in determining that.
For this issue, I propose a patch that will act as a temporary workaround and inspect ALL language type plugins to determine if any of the enabled plugins are url based.
Closed: works as designed
4.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
The current solution is correct. The problem must be fixed by a third party #3228568: LanguageNegotiationCountryPathUrl plugin not returned on Language Negotiator getNegotiationMethods() call → .