welsh plurals a very irregular. This is an example of a word where the base word is the plural i.e Moron=Carrots and Moronen=(A)Carrot. These plurals are often found for animals, plants and other things that you're more likely to come across in groups Other Examples. Llygod=Mice Llygoden=(A)Mouse Plant=Children Plentyn=(A)Child Coed=Trees Coeden=(A)Tree and most tree types too Pinwydd=Pine/Fir trees Pinwydden=(A) Pine/fir tree Derw=Oak tree Derwen=(A)Oak tree. Also worth noting is that the -en ending is almost always feminine and -yn ending almost always masculine.
Yes it's thought the Welsh comes from the Middle English more which comes from Germanic and means 'carrot', 'parsnip'.
The Swedish morot apparently comes from MIddle Low German more + wortel (which is the same as root in English).
So in short they are the same except that the Swedish has added 'root' to the end.
So now that we have established you can add rot to the end of a word to mean 'root', I have difficulty in believing that carrot is not related, especially given that Wiktionary says that the car bit is from Ancient Greek and the rot bit is unexplained. Surely it must be related.