Index: src/wp-includes/customize/class-wp-customize-media-control.php
===================================================================
--- src/wp-includes/customize/class-wp-customize-media-control.php	(revision 36363)
+++ src/wp-includes/customize/class-wp-customize-media-control.php	(working copy)
@@ -55,7 +55,7 @@
 	public function __construct( $manager, $id, $args = array() ) {
 		parent::__construct( $manager, $id, $args );
 
-		$this->button_labels = array(
+		$this->button_labels = array_merge( array(
 			'select'       => __( 'Select File' ),
 			'change'       => __( 'Change File' ),
 			'default'      => __( 'Default' ),
@@ -63,7 +63,7 @@
 			'placeholder'  => __( 'No file selected' ),
 			'frame_title'  => __( 'Select File' ),
 			'frame_button' => __( 'Choose File' ),
-		);
+		), $this->button_labels );
 	}
 
 	/**
@@ -193,8 +193,8 @@
 			</div>
 			<div class="actions">
 				<# if ( data.canUpload ) { #>
-				<button type="button" class="button remove-button"><?php echo $this->button_labels['remove']; ?></button>
-				<button type="button" class="button upload-button" id="{{ data.settings['default'] }}-button"><?php echo $this->button_labels['change']; ?></button>
+				<button type="button" class="button remove-button">{{ data.button_labels.remove }}</button>
+				<button type="button" class="button upload-button" id="{{ data.settings['default'] }}-button">{{ data.button_labels.change }}</button>
 				<div style="clear:both"></div>
 				<# } #>
 			</div>
@@ -204,7 +204,7 @@
 					<div class="placeholder">
 						<div class="inner">
 							<span>
-								<?php echo $this->button_labels['placeholder']; ?>
+								{{ data.button_labels.placeholder }}
 							</span>
 						</div>
 					</div>
@@ -212,10 +212,10 @@
 			</div>
 			<div class="actions">
 				<# if ( data.defaultAttachment ) { #>
-					<button type="button" class="button default-button"><?php echo $this->button_labels['default']; ?></button>
+					<button type="button" class="button default-button">{{ data.button_labels.default }}</button>
 				<# } #>
 				<# if ( data.canUpload ) { #>
-				<button type="button" class="button upload-button" id="{{ data.settings['default'] }}-button"><?php echo $this->button_labels['select']; ?></button>
+				<button type="button" class="button upload-button" id="{{ data.settings['default'] }}-button">{{ data.button_labels.select }}</button>
 				<# } #>
 				<div style="clear:both"></div>
 			</div>