Hopefully someone else will come and explain it far better than I can, but I think the simple answer is that "I drank" and "I have drunk" are two different tenses. There is another past tense in Welsh which equates to "I drank" (I think "I drank coffee" would be "Yfes i goffi" but my Welsh grammar is rather rusty so I might not have got that quite right)
Many languages other than English use the "I have" version of the past tense much more than we do in English and it often translates better in English in the "simple" past as in this case e.g. French would say "j'ai bu du cafe lundi" which we would translate as "I drank coffee on Monday". I would think therefore that the same applies to the translation from Welsh?
The simple past form in French has practically disappeared from the spoken language (not in literature, though), so i don't think that's a fair comparison. In peninsular Spanish, the present perfect is often used for things that happened today (He hablado con él esta mañana), though Latin America Spanish tends to match English's use a bit more closely. Still, for something that happened before today (eg. Monday), you'd use the preterite.
it appears Welsh still makes the distinction between the simple past and present perrfect but is less strict than in English. I don't know anyone who would use the present perfect in this sentence in English since the present perfect gives the idea that the time period is still going on:
-I went to the store twice this week (the week can be considered over, you won't go to the store again this week)
-I've gone to the store twice this week (the week's not over yet and i might go again)
Saying "I went to the store Monday" you know that "Monday" is over. Personally, i think the translation with have (I have drunk coffee on Monday) is just to match the Welsh construction.
English makes the same distinction between present perfect and simple past as does Spanish:
He hablado con él esta mañana / Hablé con él esta mañana
I've spoken to him this morning / I spoke to him this morning.
In the first case, it's STILL this morning; in the second, this morning is OVER.
And (more to the point, perhaps!) the same applies to Welsh:
Dw i wedi siarad ag ef y bore 'ma / Siaradais ag ef y bore 'ma.
Just to clear up a confusion over tenses. English Present perfect follows the same form as Wedi (yr amser perffaith) in welsh and passé compose in French which are classed as past tenses.
I have drunk coffee tonight
Dwi wedi yfed coffi heno
J’ai bu du café ce soir ( This is the most widely used past tense in French)
The passed simple tense
Je bus ( This past tense is only used in it’s written form in French)
The reason why they've chosen to focus on 'to have', and so early in the course, is because it's the easiest way for a beginner to use the past tense. You only need to know wedi to be able to form the past tense, using the verbs you already know. Much easier than having to learn all the different verb endings.
It may be "easier" but it's very misleading, because I have drunk (though it makes reference to a completed event) is not in the past tense; it's in the present perfect -- it's stating what the situation is NOW. The past tense form is I drank -- telling of an event that happened THEN. This is what makes "I have (NOW) drunk coffee on Monday (THEN)" a nonsense sentence.
Furthermore, on Monday (the "on" is required, in British English at least!) is not dydd Llun (which means "Monday" -- the name of the day of the week) but ddydd Llun.
"I drunk" isn't grammatical in English.
"I drank" or "I have drunk" should work, but I think we're encouraged to translate "Dw i wedi..." to "I have..."
The Welsh course is still in beta and I think it's a bit inconsistent with what alternatives are and are not allowed, so you might find simple past (e.g. "I drank") is sometimes accepted and sometimes not.
I'm not sure exactly what "beta" means for Duolingo courses specifically, but with software, games, websites, etc. when something is "in beta" or a "beta version" it means it isn't finished.
The first thing testers, or a limited number of public users might see is an "alpha version" and that is generally accepted to be very rough. Whole features are likely to missing and there might be many things that don't work.
After that comes the "beta" stage, where most things are working as intended and it's often freely open to the public, like this Welsh course. The "beta" label is there to say that not everything is finalised and some things might still need to be fixed or tweaked. It's like a final testing stage, and often means the developers are watching more closely for bug reports than they will do later on.