What helped Charles IV in his negotiations? His knowledge of five languages!
Charles IV always put the Czech language first. One of many Czech elaborate fables without any real basis. Perhaps I should just add that he did so when submitting his biography when he wanted to make money. Unfortunately, this is nonsense as he wrote his own biography in Latin only.
Source : https://jazykovyservis.cz/co-pomahalo-karlovi-iv-pri-jednanich-jeho-znalost-peti-jazyku/