Thanks for pointing out the awkward English. This default translation has now been changed to 'Yesterday, I drank coffee'.
('I had drunk...' would actually require a different tense in Welsh, the pluperfect - ro'n i wedi yfed....)
The Welsh too is awkward in my view. Should be "Ddoe yfes i goffi" since yesterday is over and done with. If want you really want is (as I suspect) to test the construction "I have drunk" then you should really choose a sentence like "Dwi wedi yfed coffi heno" (I've drunk coffee tonight), which makes sense since tonight is still ongoing.