Question: "Why do some Orthodox respond to "Christ is Risen!" with "Indeed He is Risen!" but others say, "Truly He is Risen!"? Which one is correct?"
Both responses are perfectly good translations of the responses in Greek and Slavonic. But "Truly He is Risen!" is most likely based on the Greek response, and "Indeed He is Risen!" is most likely based on the Slavonic.
The oldest English Orthodox text of the Paschal services that I have been able to find actually differs slightly from both. The Service book of the Holy Orthodox-Catholic Apostolic Church, which was translated by. Isabel Hapgood, and was originally published in 1906, and then published in a corrected edition in 1922, uses the phrase: "He is risen indeed!"
In Greek, the response is Alithos Anesti! (Αληθώς Ανέστη!), and the most natural translation of the Greek word "Alithos" would be "Truly." However, in Slavonic the response is Voistinu Voskrese! (Воистину Воскрес!), and the word "Воистину" has the prefix, "Во" which means "in" followed by "истину" which means "truth." So you could translate it literally as "In Truth," but "Indeed" is probably a more elegant way to translate it. In any case, that is how Isabell Hapgood translated it, and although we did not keep her phrasing exactly, it probably influenced the form we now commonly use.
I hope someone writes a good book on the history of English translations of Orthodox liturgical texts, because you can see that usage has evolved. For example, Hapgood translated "Theotokos" as "Birth giver of God," which is a good literal translation, but most English speaking Orthodox today simply use "Theotokos," which has been in English usage as theological term since at least 1868. On the other hand, it is interesting that Hapgood's translation of the Paschal Troparion ("Christ is risen from the dead, trampling down death by death, and upon those in the tombs bestowing life") is what is most commonly used today. So over time, what seems to work best in English bubbles to the surface, and we settle on particular translations.
