Hi Juraj,
Thank you for your response.
Yes, the xlf file contains both the source and target data. It looks like this:
<xliff version="1.2">
<file original="cms.document;1822" source-language="de-DE" target-language="en-GB" datatype="htmlbody">
<body>
<trans-unit id="title">
<source><![CDATA[Das ist ein Test.]]></source>
<target><![CDATA[This is a test.]]></target>
</trans-unit>
...
Unfortunately, the error is not logged to the Event log application.