can't set livechat offlineSuccessMessage

Description:

I can't set a string for offlineSuccessMessage neither by admin backend nor RocketChat(function()

Steps to reproduce:

I'm setting a language (german) for the Livechat Widget API
and set the offlineSuccessMessage with:

RocketChat(function() {
this.setLanguage('de');
this.setTheme({
offlineSuccessMessage: 'Ihre Nachricht wurde versendet',
});

Expected behavior:

The language changes fine, but he offlineSuccessMessage is still default, english.

Actual behavior:

In the admin backend is a field, where you are able to set another text for the offlineSuccessMessage
that doesn't work.
That's why I also tried to insert
this.setTheme({
offlineSuccessMessage: 'Ihre Nachricht wurde versendet',
});

Server Setup Information:

• Version of Rocket.Chat Server: 3.12.0
• Operating System: Ubuntu
• Deployment Method: self ssh
• Number of Running Instances: 1
• NodeJS Version: v12.20.0
• MongoDB Version: 4.0.19 / mmapv1 (oplog Aktiviert)

Start with the Livechat Widget API language and theme entry points, then reproduce the issue using the RocketChat(function) calls shown in the report and the admin backend setting. Done means offlineSuccessMessage uses the configured German text instead of the default English message in both configuration paths.