Harness of language?

Hello, I want to ask for help about a collocation that I'm not quite sure about. It's in a recommendation letter.

The sentence is:

Her very first English public speech in our training session showed her outstanding harness of languages and effective interactions with the audience.

Here I want to say she has a good control and usage of language in her speech, but I didn't find the collocation like "harness language". But I feel that harness should be the proper word, can anyone help me to check if this makes sense??

Thank you very much!!!