Add Icelandic language to UI

app/soapbox/features/preferences/index.tsx

@@ -38,6 +38,7 @@ const languages = {
   hy: 'Հայերեն',
   id: 'Bahasa Indonesia',
   io: 'Ido',
+  is: 'íslenska',
   it: 'Italiano',
   ja: '日本語',
   ka: 'ქართული',

app/soapbox/locales/messages.ts

@@ -58,6 +58,7 @@ const locales: string[] = [
   'hy',
   'id',
   'io',
+  'is',
   'it',
   'ja',
   'ka',