In that great song "We'll Keep a Welcome in the Hillside" there is one Welsh word. What is it and why is it not written in English?

The word is 'Hiraeth', and it is not written in Emglish because there is no direct English translation.

One site says:

The word "Hiraeth" doesn't have a direct translation. It means "longing" but much more so. It's the sort of longing to be home or with someone which is so strong it makes you ill and stops you sleeping. The lyrics of the song probably explain it better than I can.

Hiraeth is the bond that keeps the Welsh together wherever we may be and cannot be understood by someone who is not Welsh,we are born with it and is always with us.I'll have a word with Maggie and see if you can be given honorary Welsh citizenship.This confers the priviledge of running around naked in Druid ceremonies on St David's Day,singing SOSPAN FACH at the top of your voice,and being on the losing side at rugby matches.On the negative side,many people will be jealous of you,but you take it in good faith they can't all be blessed like the Welsh