The Welsh alphabet is not the same as the English one. There are eight Welsh letters that are written as pairs of characters in Modern Welsh, and ll is one of them - it is a separate letter from l.
Watch the video that we recommend for a good introduction to the alphabet - https://www.youtube.com/watch?v=Gb8Bps3bG84feature=youtu.be
You will find another useful alphabet table together with the IPA symbols for the basic pronunciation of the Welsh letters if you look on the web for the article 'Omniglot Welsh alphabet'.
I thought: llaeth = milk ... but wait, Nain taught me llefrith (not sure of spelling) is milk. So, can’t be milk but what is it? Checked the word and it’s milk. ... ... so, i’ve been wondering: is it generally true to say that the south welsh word is likely to be the older one? Hoffi etc being older than licio and there’s another word for trio (sorry don’t know spelling), etc. ??? I can imagine that in the parts of wales where the language was killed off more successfully they might have chosen to learn the older, more obviously welsh word for things when they got it going again. ... That’s probably not right though as my own family lost it in my great grandmother’s generation in north wales and so, although my Nain and her Nain a Taid had Cymraeg as first language, her mother had to teach herself as an adult, like me now.
You can look up historical examples of word usage in the on-line Geiriadur Prifysgol Cymru.
The idea of 'north' and 'south' in Welsh dialects is a gross over-simplification of the actual state of things. Have a look at the course notes for the section 'Dialects' for some links to more information on the 4-6 main dialects in Wales. None of the current dialects can really claim to be older than any of the others. Quite a lot of dialect words are not in the standard dictionaries because they are not often seen in writing. You will come across booklets on some local dialects, so it is worth keeping an eye out for anything about your own local area.
"Llaeth" is the base word for milk. Sometimes it can be "Laeth" but only when something causes it to become that e.g "Dy laeth" (Your milk) where the "Dy" causes what's called a soft mutation. Here there is nothing to cause it to change.