Upgrade/Install: Provide a default label for "Continue" button in language selection form.

Prevents an empty continue button if a translation isn't yet available.

Props sathyapulse.
Fixes #48776.
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git-svn-id: https://core.svn.wordpress.org/trunk@46584 1a063a9b-81f0-0310-95a4-ce76da25c4cd