Index: src/wp-admin/includes/ms.php
===================================================================
--- src/wp-admin/includes/ms.php	(revision 53038)
+++ src/wp-admin/includes/ms.php	(working copy)
@@ -1017,6 +1017,7 @@
 		// as there is nothing to download.
 		if ( ! languageSelect.find( 'option:selected' ).data( 'installed' ) ) {
 			$( '#submit', this ).after( '<span class="spinner language-install-spinner is-active" />' );
+			$( '#submit', this ).attr( 'disabled', true );
 		}
 	});
 } );
Index: src/wp-admin/includes/options.php
===================================================================
--- src/wp-admin/includes/options.php	(revision 53038)
+++ src/wp-admin/includes/options.php	(working copy)
@@ -94,6 +94,7 @@
 			// as there is nothing to download.
 			if ( ! languageSelect.find( 'option:selected' ).data( 'installed' ) ) {
 				$( '#submit', this ).after( '<span class="spinner language-install-spinner is-active" />' );
+				$( '#submit', this ).attr( 'disabled', true );
 			}
 		});
 	} );