Log the context of the ULS in which no search results were found

Relies on updates to EventLogging schema. Diff:
https://meta.wikimedia.org/w/index.php?title=Schema%3AUniversalLanguageSelector&type=revision&diff=17799034&oldid=7327441

Bug: T179402
Change-Id: I2ff9ce9b40df16ed3c5e6970fb3b691af08cedec

resources/js/ext.uls.displaysettings.js

@@ -356,6 +356,7 @@
 					// the apply button
 					displaySettings.markDirty();
 				},
+				ulsPurpose: 'interface-language',
 				quickList: function () {
 					return mw.uls.getFrequentLanguageList();
 				}