As Wendy gives Vicky a bath she says: 'Dere i fi olchi dy gefn di'. 'Dere' is the normal way of saying 'come on' in South Wales. But here 'dere' is followed by 'i fi' and a verb' - which is of course mutated! Listen again. 'Dere i fi olchi dy gefn di'. Literally this translates as: 'Come for me to wash your back'. It's the equivalent of saying 'Let me wash your back'.
You could also use this pattern to say 'Let's go' - Dere i ni fynd. or 'Let me see' - Dere i fi weld.

In one of their angry exchanges, Brian says to Wendy:

Brian - Glywist ti be ddudish i?
which means 'Did you hear what I said?'

Glywist ti and dudish i are the short past tense forms of the verbs 'clywed' and 'dweud'.
The formal past tense ending when referring to oneself is '-ais' , as in dywedais i - 'I said', but Brian, in his northern dialect says '-ish' - as in 'dudish i'.
Other examples are:

gwelish i - I saw
prynish i - I bought

The phrase for 'oh, poor you' in Welsh is 'o, druan ohonot ti', as Michelle says to Brian, when she sympathises with him for all his problems.

'Truan' means 'poor' in the sense of 'wretched' or 'miserable' and must not be confused with 'tlawd', which means 'poor' in the sense of being short of money. It may interest you to know that the word 'truant' is one of the few English words borrowed from Welsh. Yes, it makes you think of 'truants' in a different light, doesn't it?. They are unhappy people who can't cope, not just lazy ones who can't be bothered turning up to school or work.