When a file is exported in ITAM it is encoded as UTF-8 without BOM (byte order mark). As a result German characters such as Umlauts (i.e. ä, ü) as well as other languages' accents (á, î, à) are not displayed correctly in the exported CSV file.
In order to enable these special characters in ITAM you should edit the file web.config at ...\CA\ITAM\Application Server and update the entry for Export encoding as below:
<add key="ExportCSVEncoding" value="UTF32"/>
After making the change you should reset IIS services and also restart CA APM Export Service via the Services Management console.