Multilanguage page title
Here are a few pointers which might help:


If you are talking about the name of your site, then please use the Admin > Configuration > System > Site information page to configure it - once in English and once in German.
If you are talking about a page node you created, then:


Make sure that you enable translations for the title property from content translation settings.
Make sure you use the Translate tab on the node to input the German title for your page.

Having done everything above, if you still do not see the German title on the German version of your page, try clearing cache from the Configuration > Development > Performance page.


References:


https://www.drupal.org/docs/8/multilingual/translating-content