From what I understand from the website, you want (and I provide, in Welsh):

hello - same as 'how are you'
how are you? - Sut wyt ti? (informal & singular) Sut ydych chi? (sing. formal and plural)
welcome - croeso
goodbye - 'Da bo (short for Duw bo gyda ti / chi, God be with you)
please - Os gweli di'n dda (informal & singular) Os gwelwch yn dda (sing. formal and plural)
what is your name? - Beth yw dy enw? (inf & sing) Beth yw eich enw (sing form & pl)
my name is ... - ... yw fy enw i.
Do you speak English - Wyt ti'n siarad Saesneg? (inf & sing) Ydych chi'n siarad Saesneg? (sing form & pl)
Yes - no simple form; depends on how the question is up
No - usually depends on the question, but you can get away with Na or Nage



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siân