Translation:We just arrived today.
"We have just arrived today" should be the actual solution. According to the British Council: https://learnenglish.britishcouncil.org/en/quick-grammar/just-yet-still-already
‘Just’ is usually used only with the present perfect tense and it means ‘a short time ago’.
I’ve just seen Susan coming out of the cinema. Mike’s just called. Can you ring him back please? Have you just taken my pen? Where has it gone? In the present perfect, ‘just’ comes between the auxiliary verb (‘have’) and the past participle.
I know that many American English speakers use the simple past tense with 'just' but the best answer should have the 'present perfect' as mentioned before.
In many sentences the present perfect is now accepted, though the incorrect past simple is still the 'official' translation (thank you to whoever is checking and correcting!), but it's still not accepted in this sentence 2019-08-31. It pained me to write 'We just arrived' only to finish this unit.
(I have never heard of anyone saying such a sentence without the 'have' though...lol) It is exactly 'today' which emphasises the time point at which we arrived, thus the perfect tense should be adopted. Grammatically, anything otherwise would barely make any sense (at least in BrE?)