Fix non-constant variable definition (ampproject#6627)

translation can be overwritten if the language isn't recognized.

examples/source/1.components/amp-script/index.html

@@ -507,7 +507,7 @@ <h2 id="translated-greeting"></h2>
         };
 
         const lang = navigator.language.slice(0, 2);
-        const translation = translationMap[lang];
+        let translation = translationMap[lang];
         if (!translation) {
           translation = "Couldn't recognize your language. So: Saluton";
         }