GH-5847 Add missing localization string (#6130)

* Add missing localization string to channel_intro_messages.jsx

* Update en.json

* Fix indentation

webapp/i18n/en.json

@@ -1542,6 +1542,7 @@
   "integrations.outgoingWebhook.description": "Outgoing webhooks allow external integrations to receive and respond to messages",
   "integrations.outgoingWebhook.title": "Outgoing Webhook",
   "integrations.successful": "Setup Successful",
+  "intro_messages.channel": "channel",
   "intro_messages.DM": "This is the start of your direct message history with {teammate}.<br />Direct messages and files shared here are not shown to people outside this area.",
   "intro_messages.anyMember": " Any member can join and read this channel.",
   "intro_messages.beginning": "Beginning of {name}",

webapp/utils/channel_intro_messages.jsx

@@ -150,6 +150,13 @@ export function createDMIntroMessage(channel, centeredIntro) {
 }
 
 export function createOffTopicIntroMessage(channel, centeredIntro) {
+    var uiType = (
+        <FormattedMessage
+            id='intro_messages.channel'
+            defaultMessage='channel'
+        />
+    );
+
     return (
         <div className={'channel-intro ' + centeredIntro}>
             <FormattedHTMLMessage
@@ -159,7 +166,7 @@ export function createOffTopicIntroMessage(channel, centeredIntro) {
                     display_name: channel.display_name
                 }}
             />
-            {createInviteChannelMemberButton(channel, 'channel')}
+            {createInviteChannelMemberButton(channel, uiType)}
             {createSetHeaderButton(channel)}
         </div>
     );