Note: After saving, you have to bypass your browser's cache to see the changes. Google Chrome, Firefox, Microsoft Edge and Safari: Hold down the ⇧ Shift key and click the Reload toolbar button. For details and instructions about other browsers, see Wikipedia:Bypass your cache.
// below code is copied and modified from MediaWiki:Gadget-citations.js
var $spinnerspan;
var cbButton;

// Add a reference translation button next to the "Show changes" button
function addTrRefButton() {
	if ( document.getElementById( 'wpDiff' ) ) {
		var diff = document.getElementById( 'wpDiffWidget' );
		cbButton = new OO.ui.ButtonWidget();

		cbButton.setElementId( 'wpTrans' )
		.setLabel( 'Translate refs' )
		.setFlags( 'secondary' );

		cbButton.on( 'click', function () {
			$.ajax( {
				timeout: 600000, // thousandths of a second, so 10 minutes
				type: 'POST',
				url: 'http://localhost:19050',
				data: {
					text: $( '#wpTextbox1' ).textSelection( 'getContents' ),
				},
				beforeSend: function() {
					$( cbButton.$element ).replaceWith( $spinnerspan );
				},
				success: function( data ) {
					expandedText = data.expandedtext;
					$( '#loader' ).replaceWith( cbButton.$element );
					if ( expandedText ) {
						// Populate text box with received expanded text
						$( '#wpTextbox1' ).textSelection( 'setContents', expandedText );
						// Populate edit summary textbox with received edit summary
						$( '#wpSummary' ).textSelection( 'setContents', editSummary );
						// Click 'Show preview' button
						$( '#wpDiff' ).click();
					} else {
						$( '#loader' ).replaceWith( cbButton.$element );
						alert( 'Error: Replacement text empty' );
					}
				},
				error: function( data ) {
					$( '#loader' ).replaceWith( cbButton.$element );
					alert( 'Error: Translation request failed' );
				}
			} );
		} );
		
		cbButton.$element.insertBefore( $(diff.nextSibling) );
	}
}

// Add a button to allow translating references in the text in the edit textbox
mw.hook( 'wikipage.editform' ).add( function() {
	if( !$spinnerspan ) {
		$spinnerspan = $( '<span id="loader"></span>');
	}
	if ( document.images ) {
		// Loading gif when Translation request is processing
		var spinner = new Image();
		spinner.src = "//upload.wikimedia.org/wikipedia/commons/4/42/Loading.gif";
		$spinnerspan.append( spinner );
	}
	if( $('#wpTrans').length === 0 ){
		mw.loader.using( [ 'oojs-ui-core', 'oojs-ui.styles.icons-content', 'jquery.textSelection' ] ).then( addTrRefButton );
	}
} );