Converting numbers 1-99 to Spanish in Javascript
The function declarations are at least lacking a const statement (or let or var if you prefer):

const getSpanishCardinal = cardinal =&gt; {


However I would suggest to using regular function statements instead.



The getCardinals function is a bit pointless. A simple constant instead would make more sense:

const CARDINALS =  {
  0: "cero",
  1: "uno",
  // ...
}


For readability I'd also suggest to split it up into separate objects for 0 to 15, the tens, the hundreds, etc. 

This would also allow to assign assign the tens to the base number e.g.: 

const TENS = {
     2: "veinte",
     3: "treinta",
     4: "cuarenta",
     // ...
}


and then you don't need to "build" numbers in leftMostMultiplier:

const twentyToNinetynine = cardinal =&gt; {
  // ...
  return (
    TENS[cardinal / 10] +
    " y " +
    getSpanishCardinal(rightMostDigit(cardinal))
  );
};


(BTW, there seem to be a lot of unnessecary toString()s.)