The requirements here appear out of date: https://docs.microsoft.com/en-us/azure/cognitive-services/translator/dynamic-dictionary

I just ran test for using the dynamic dictionary, using the following test sentence: "Das Wort <mstrans:dictionary translation="wordomatic">word or phrase</mstrans:dictionary> ist aus dem Duden."

This works fine, e.g. when translating into French with and without specifying the source language (with "from").

Therefore I believe the following above-stated requirements are both out of date:

The From and To languages must include English and another supported language. (here, the source is German)

You must include the From parameter in your API translation request instead of using the autodetect feature. (German is auto-detected and dynamic dictionary still works)