What does "swear polyglot" mean?

Note that this is not idiomatic English. The speaker, Van Helsing, is Dutch, and Stoker characterizes him with odd approximations to English, which are supposed to be funny and occasionally are.

Polyglot's easy—as bib and Mark Thorin tell you, the captain swore in many languages. Bloom and blood is made clear earlier in the passage:

. . . “Final the captain, more red than ever, and in more tongues tell him that he doesn’t want no Frenchmen—with bloom upon them and also with blood—in his ship—with blood on her also. And so, after asking where there might be close at hand a ship where he might purchase ship forms, he departed.
   “No one knew where he went ‘or bloomin’ well cared,’ as they said, for they had something else to think of—well with blood again; for it soon became apparent to all that the Czarina Catherine would not sail as was expected. A thin mist began to creep up from the river, and it grew, and grew; till soon a dense fog enveloped the ship and all around her. The captain swore polyglot—very polyglot—polyglot with bloom and blood; but he could do nothing.”

Your sentence thus means:

The captain swore in many languages, quite a number of languages, with frequent repetition of the words bloomin’ and bloody; but he could do nothing

Literally : swearing again and again, in many languages.

The meaning is rather is that he was not satisfied with only one swear-word, but poured out a stream of them, like Capitaine Haddock by Hergé.