This episode is the first time that the word 'Viking' is used in the series.
Yet another antiquated language is used in the series as in this episode, "Frankish" or "Old Franconian" is spoken by the character named Sinric.
The language of France during the Viking invasion of Paris in the 9th century would have been Old Frankish, a West Germanic Language. It would have sounded more German than Latin or modern French. Charles the Bald was born in Frankfurt, he would have grown up speaking some form of German.