As with other languages, different dialects have developed over the centuries (five-ish main dialects in Wales) and in this case we have ended up with several ways of pronouncing the word and two different spellings as well.
We also have half a dozen or so words for each of boy, girl, hedge and gate, for example, so having two for milk is nothing to be surprised by!
Regional preferences in word use -- much as English speakers say "torch" versus "flashlight" or "tap" versus "faucet" depending on where they are from.
UK English speakers would never say flashlight or faucet, they are American English.
Isn't it a bit odd switching between llefrith and llaeth without explanation? I used llaeth in the first half and had to guess what llefrith was milk.
Ive finished this section and am doing daily practice. However "llefrith" never appeared in the lessons, only "llaeth", so shouldn't be used here imo.
There is no reason not to allow it. Anyone who is learning Welsh with a face-to-face class in North Wales, or who is using other materials along with Duolingo might know "llefrith" (Northern Welsh) rather than "llaeth". If you have learned "llaeth", use "llaeth", but you need to be aware that in conversation or if you're reading in Welsh, people may use "llefrith".
Either word should be accepted for every occurence of 'milk'.
Note that there are two or three main dialects in what people think of as 'north' Wales.
It's like an "unvoiced L" -- a sound we don't have in English (or many other languages). If you like analogies, "unvoiced L" is to L as S is to Z.
Try saying "ssss zzzz ssss zzzz" or "ffff vvvv ffff vvvv". The difference between S and Z is voicing. If you put your hand on your throat, you'll feel vibration for the Z, but not for the S. Other than voicing (vibration), S and Z are exactly the same. Same goes for F and V.
Now say the L sound LLLL. Feel the voicing? Now turn off the voicing -- stop vibrating your throat -- but don't change the position of your tongue, or anything else. If you do it right, you'll end up with a perfect Welsh ll.