Duolingo's “Golden Rule of Translation” is not about “functional equivalents” but says “translate as literally as reasonably possible and only as freely as necessary.” For learning a language this seems to be a very good rule. Otherwise, how is Duolingo supposed to know that you recognized aniversăm as a verb and ani as a noun meaning “years?”
Moreover, there might be people who do not celebrate their anniversaries, so your sentence is not even a “functional equivalent.”
I take your point about the grammar, and being an English teacher I'm quite keen on people getting the grammar right. But as virtually no native speaker would say ''We celebrate ten years", for me Duo's translation here lies outside the bounds of what is "reasonable".
But if the verb here does actually means "celebrate", as opposed to "have an anniversary" then perhaps:
"We're celebrating our tenth anniversary"
would be closer to the original. Point taken.
Incidentally, I've been using Duo for six years over a variety of language combinations and pretty much follow that rule to the letter. But there are some cases where sentences have to be changed to make sense in the other language (this being one of them), and in these cases, Duo not only usuallt accept functional equivalents, they often actively teach them, including even in this course.
Yes, you are right. " sărbătorim" = when a normal ongoing day is disrupted by some sort of festivity or party, that are honoring a person or event. Ex: " Sărbătorirea Zilei Muncii=Celebrating Labor Day", " Sărbătorirea Zilei Naționale= Celebrating National Day", " sărbătorirea zilelor religioase= celebrating religious days" and " sărbătorirea zilelor de naștere = celebrating birthdays ", etc.
" aniversăm"= is when a certain day reminds us of something important that happend one or more years ago. Ex: " aniversarea căsătoriei= Wedding anniversary", " aniversarea Revoluției din România= anniversary of Romanian Revolution" ," aniversarea Zilei Unirii Pricipatelor Române= the anniversary of Romanian Unification Day", etc.