Language configuration and translation assignment
Check translation mapping, visibility and availability of both language pages. A translation that can be achieved only in one direction does not result in a complete pair of languages.
Compare site languages, basic URLs, and the translations that actually exist. A detail extension must assign the same data sets and accessible paths as the output language references. Language fallback and real translation are not always the same content.
Where to check the setting in TYPO3
Control the languages of the site configuration and the translation relationship of the page or dataset. hreflang must match accessible language versions. A fallback output with unchanged content is not automatically an independent translation.
Example of this specific finding
Illustrative control case: The German side calls the English version, which knows only itself. The language set is incomplete.
follow-up audit in TYPO3
Open the other versions from each language version and check their canonical and reference back. Do not replace an empty or unavailable translation with a foreign homepage.
Retrieve the assignment from both URLs and supplement backlinks. For multiple languages, check the entire set instead of just a single pair.
Derogations and scope
A link in the voice menu does not replace the hreflang assignment. Both can come from different data sources.
Reference: TYPO3 13.4 with EXT:seo. For other major versions and own extensions, responsibilities may differ; check the actually installed output.
Classify the general error
Definition, system-independent causes and further correction examples: Check language references as a related group