Add PDF viewer label in

services/web/app/views/project/editor/left-menu.jade

@@ -127,7 +127,7 @@ aside#left-menu.full-size(
 						option(value=size) #{size}px
 
 			.form-controls
-				label(for="pdfViewer")
+				label(for="pdfViewer") #{translate("pdf_viewer")}
 				select(
 					name="pdfViewer"
 					ng-model="settings.pdfViewer"