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The message key 'Wikibase-unconnectedpages-format-row' needs plural support.

Current message contents are:

($1 interlanguagelinks on the page)

This should be:

($1 interlanguagelink{{PLURAL:$1||s}} on the page)

The case of $1 being 1 seems grammatically incorrect in English, but not explicitly adding plural support confuses translators. Having "No interlanguage links on the page" could also be added in the same message (MediaWiki 1.21 and later required). In that case, you could consider using the following:

({{PLURAL:$1|$1 interlanguagelink|$1 interlanguagelinks|0=no $1 interlanguagelinks}} on the page)