Zotero does not automatically recognize languages, you may need to provide specific PDFs to easily determine if it's a data error in the database or something else. Also, there are a lot of plugins that offer added languages, try troubleshooting them and if it's the plugin, report it to the plugin author.
northword said, "Zotero does not automatically recognize languages..."
Zotero's language tag is received from metadata provided by the publisher of the journal or database. Several publishers do not provide a language tag and Zotero, properly, doesn't impute one. Some non-English journal publishers will provide an "en" tag with the English abstract page even when the article itself is not in the English language.
The recommendation to view the PDF can be the only way to know the true language of the full article.
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Zotero's language tag is received from metadata provided by the publisher of the journal or database. Several publishers do not provide a language tag and Zotero, properly, doesn't impute one. Some non-English journal publishers will provide an "en" tag with the English abstract page even when the article itself is not in the English language.
The recommendation to view the PDF can be the only way to know the true language of the full article.