Boil em, mash em, stick em in a stew...
Is it borrowed from German Kartofell ?
Yes! It's from German "Kartoffel" which in turn is borrowed from Italian "tartuficolo". That word is inherited from the Latin "territūberum". So ultimately it has a Latin origin! Word etymologies are fascinating...
for the listening exercises, is there an easy way to tell if it is singular or plural?
Not really. While I think the TTS is generally pretty bad, the "-i" at the end sounds ok in this case. You just have to get used to palatalization at the end of words.
What is this palatalization? What am I listening for in the different pronunciations?