Language chooser imporvements.
 * Use a translated 'Continue' string.
 * Go back to using a select element.
 * Only show a spinner when we're installing a language. 

see #28577.

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git-svn-id: http://core.svn.wordpress.org/trunk@29150 1a063a9b-81f0-0310-95a4-ce76da25c4cd