Probably, yes. Did you report it using a "my answer should be accepted"? That is the quickest way to get the team to look at it.
Technically the above is a valid translation too. I can think of expressions where I would definitely, as a native English speaker, say "I have had..." For instance, "I have had enough of this nonsense". In Welsh that is "dw i wedi cael llond bol o'r lol hyn".
I think they should just accept either here.
Now you mention simple past, I have to rethink!
Wedi is always used to form the perfect tense (present perfect, future perfect, conditional perfect and pluperfect). "Dw i wedi" is the (present) perfect, and cael is the verb "to have" that is being placed into the perfect tense. I thought "yesterday I had coffee" was fine because it is clear that this is a finished action, but now I think about it, it is not strictly the present perfect. "I have had coffee" is correct here. That differs subtly from simple past, for which you don't use wedi.
Simple past is denoted by conjugating the verb, or using "bod" in the simple past (a tense that English does not have, but there is a lesson on it here).
Ces i goffi is "I had coffee" and I retract my agreement that "I had coffee" is a better translation here. It does literally mean "I have had coffee". Sorry!
You're right. "Yesterday, I have had coffee" isn't good phrasing in any variety of English I know of. Since yesterday is in the past, the past perfect is an option, but only if you want to describe some sequence of events or give more time constraints on something that you finished doing within the previous day, as in "Yesterday I had had coffee before eating breakfast" or "Yesterday I had had coffee by noon."
On the other hand, I definitely agree it's important to know that Welsh uses present (? actually, I think wedi is an infinitive, but I don't know) perfect in this context, so it makes some sense that they also give the literal translation (if that's really what's going on). I think what would help is if there were a heads-up in the lesson's notes.
For what it's worth, for contrast, I wonder how, "Yesterday I had had coffee by noon" and "Yesterday I had had coffee before eating breakfast" would be said in Welsh.
Edit: This got addressed here. Apparently wedi is a preposition meaning after, so I guess these sentences just need to be fixed in English. And I guess the past perfect in Welsh might be rendered like, "I was after..." "Roeddwn i wedi..." maybe.
I get that this sentence is grammatically correct, but it doesn't make any logical sense. "I have had coffee" and "Yesterday, I had coffee" work fine, but this just doesn't sound natural. In English, the perfect tense denotes in-specificity. You can't say "30 minutes ago, the woman has given birth," because you are giving an exact point in time. I don't understand how the mods could have given such a nonsensical translation
"it doesn't make any logical sense"
Yesterday I am after having a coffee is the literal translation, and I agree with you that it sounds nonsensical.
However, I am after having a coffee yesterday. makes much more sense. Why? Because here it is clear that "yesterday" modifies "having a coffee", whereas is in the first sentence it seems to modify "am".
It's the same in English: In I have had a coffee yesterday, the "yesterday" clearly modifies "had a coffee", but in Yesterday I have had a coffee, it seems to modify "have". It is obvious though that it is not "have" which is being modified, because that would not make sense (it would have to be "had"). Speakers of other languages which use this sort of construction for the present-perfect, such as Welsh, German and French, are used to this and so it does not seem weird to them, but English speakers usually switch to the simple present when extra details are given which is why it sounds weird to us: we are not used to this structure.
As for why the mods gave this translation, I guess they want to make clear that the past-perfect and simple past have different – though subtly different – meanings, whilst acknowledging that "Yesterday I have had a coffee" is unnatural English.
And I understand how common this is in other languages, but we are translating this sentence into English and not those other languages. Yes, in German the preterite has fallen out of general use, so the most acceptable translation here would be "Gestern hab ich einen Kaffee gehabt." However, this is not how English grammar works. You simply cannot use the present perfect with a specific point of time. Languages to not always translate literally and when they do not, you have to modify. Right now we are talking strictly about the English sentence, which is nonsensical. I understand the the Welsh is correct. Finally, the mods do not acknowledge that "Yesterday I have had coffee" is unnatural in English