Use the catalan name for the default city (Eivissa).

data/org.gnome.shell.extensions.openweather.gschema.xml.in

@@ -57,7 +57,7 @@
       <_description>Choose whether to display wind direction through arrows or letters.</_description>
     </key>
     <key name="city" type="s">
-      <default>'2516479>Ibiza (ES)'</default>
+      <default>'2516479>Eivissa (CA)'</default>
       <_summary>City to be displayed</_summary>
     </key>
     <key name="actual-city" type="i">

src/extension.js

@@ -705,7 +705,7 @@ const WeatherMenuButton = new Lang.Class({
             cities = [];
 
         if (cities.length == 0) {
-            this._cities = "2516479>Ibiza (ES)";
+            this._cities = "2516479>Eivissa (CA)";
             this.updateCities();
             return;
         }