Translation:Duolingo said my translation was wrong.
Yep, in reported speech, the tense of the whole sentence changes when the main verb is in past tense.
Beg to differ.. What Duolingo said is past tense. Hence "said" is correct but it does not change the fact that the translation is wrong (if it truly is). Therefore, "is" should be used instead.
Wilson Ta is right. Reported speech doesn't change the tense of the verb to past tense. Take the following examples: He said the earth is flat. She said we will be unable to contact her.
Both "was" and "is" are fine in informal writing.
Technically, the rule of "the sequence of tenses" means that "was" is more correct unless it's verbatim and in quotation marks.
Of course, we don't use quotation marks in spoken English.
To add to that, the fact remains (e.g. that the translation is wrong) hence it cannot be in past tense.
"Duolingo app said my translation was incorrect" was marked wrong, even though it was correct.