I tried the same thing, but it looks like 'mynd i' as 'to go to' is distinct from the more general future tense. Google gives me "Byddaf yn yfed cwrw" for 'I will drink beer', and I see bydd listed in the words for the Future1 skill a ways down the tree (https://www.duolingo.com/comment/19825204). On the other hand, Google also gives me "Yr wyf yn mynd i yfed cwrw" for 'I am going to drink a beer', so it may be working from a slightly different script.
Google also gives me "Yr wyf yn mynd i yfed cwrw" for 'I am going to drink a beer', so it may be working from a slightly different script.
Yes, Google is using the formal language there. Byddaf yn yfed cwrw is more formal too. Google Translate isn't very good at distinguishing different registers of the language.
In view of this Welsh application of 'mynd i', I wonder if the use in both English and French of the verb 'to go' to express purpose or futurity (English 'I am going to drink', French 'je vais boire') is a relic of the Celtic past. Neither language is Celtic, of course, but both France (ancient Gaul) and Britain were one Celtic-speaking; could this have left a mark on their present languages? Is 'mynd i' in this sense found in early Welsh?