The suggest Rest Service (/rest/jpo/1.0/parent/suggest) needs the following json as payload:

{"query":"","issueTypeName":"Epic","maxResults":100}

Jira's Rest-services return the issue type name in the user's language (changed within: https://jira.atlassian.com/browse/JRASERVER-27113)

If you use the German issue type name,

{"query":"","issueTypeName":"Episch","maxResults":100}

no results will come back.

The suggest Rest Service does not use the user's language. It uses internally, probably, the default language of the jira system.

The service should:
first search with the default language and then with the language of the user.
or
you should be able to use the issue type id.

{"query":"","issueTypeId":5,"maxResults":100}