[Xrystal]

What does it do ?
What did you expect it to do ?

It doesn't work doesn't explain what is happening in your case.

Looking at your code you are still only setting the information on PLAYER_LOGIN. This means it will only show the status ( outside of the printed statements from my code portions) when you first login or reload the UI.

Your updating of the display code needs to be updated on completion too.

Give this code a whirl. It's not perfect but hopefully it will work closer to what you wanted it to do.

local addonName, addon = ...
 
-- Create Frame
local frame = CreateFrame("Frame", "QuestCompletionFrame", UIParent)
frame:SetSize(100, 100)
frame:SetPoint("CENTER")
frame:EnableMouse(true)
frame:SetMovable(true)
frame:RegisterForDrag("LeftButton")
frame:SetScript("OnDragStart", frame.StartMoving)
frame:SetScript("OnDragStop", frame.StopMovingOrSizing)
 
local text = frame:CreateFontString(nil, "OVERLAY", "GameFontNormal")
text:SetPoint("CENTER")
 
-- Initialise Data used in addon
local completedQuests = {}
local questInfo = {}
local questIDs = {70893, 71995}
 
-- Setup Localization
local localization = {
    ["enUS"] = {
        ["quest70893"] = "|cFFDEB887Community Feast:|r",
        ["quest71995"] = "|cFFDEB887Trial of the Elements:|r",
        ["yes"] = "|cFF1EFF00Complete|r",
        ["no"] = "|cFFDC143CIncomplete|r"
    }
}
local locale = GetLocale()
local locStrings = localization[locale] or localization["enUS"]
 
local function UpdateDisplay(questID, questStatus)
 
    -- Append newline if questStatus contains information
    if questStatus != "" then
        questStatus .. "\n"
    end
    
    -- Convert questID to a string and create a localization key for it
    questStatus = locStrings["quest" .. tostring(questID)] .. " "
    
    -- Add whether it is completed or not
    if completedQuests[questID] then
        questStatus = questStatus .. locStrings["yes"]
    else
        questStatus = questStatus .. locStrings["no"]
    end
 
    -- Update Text Box
    text:SetText(questStatus)
 
    -- Return the current questStatus contents
    return questStatus
 
end
 
-- Handle what to do when you first log in
local function PlayerLogin(...)
    for _, questID in ipairs(questIDs) do
        local isCompleted = C_QuestLog.IsQuestFlaggedCompleted(questID)
        completedQuests[questID] = isCompleted
    end
    
    -- Statu with an empty questStatus and update it
    local questStatus = ""
    questStatus = UpdateDisplay(70893, questStatus)
    questStatus = UpdateDisplay(71995, questStatus)
 
end
 
 
-- Handle what to do when you turn in a quest
local function QuestTurnedIn(...)
    local questID, xpReward, moneyReward = ...
    
    -- Don't bother continuing if we are not watching this quest
    if questID != 70893 then return end
    if questID != 71995 then return end
    
    local isCompleted = C_QuestLog.IsQuestFlaggedCompleted(questID)   -- Returns true
    completedQuests[questID] = isCompleted
    
    -- Debug Text
    print("Quest: ", questID,"XP: ", xpReward,"Money: ", moneyReward,"Completed: ",isCompleted)
    
    -- Get Current Text Display and Update it
    local questStatus = text:GetText()
    questStatus = UpdateDisplay(questID, questStatus)
 
end
 
-- Handle what to do when you accept a quest
local function QuestAccepted(...)
    local questID = ...
    local questLogIndex = C_QuestLog.GetLogIndexForQuestID(questID)
    local info = { C_QuestLog.GetInfo(questLogIndex) }
    
    -- Add Quest Info to table 
    questInfo[questID] = info
    
    -- Debug Text
    print("Accepted Quest: ", questID, info["title"])
end
 
-- Handle events being listened to 
frame:SetScript("OnEvent", function(self, event, ...)
    if event == "PLAYER_LOGIN" then
        PlayerLogin(...)
    elseif event == "QUEST_TURNED_IN" then
        -- Triggers when pressing Complete Button
        QuestTurnedIn(...)
    elseif event == "QUEST_ACCEPTED" then
        -- Triggers before Pressing Accept Button
        QuestAccepted(...)
    end
end)
 
frame:RegisterEvent("PLAYER_LOGIN")
frame:RegisterEvent("QUEST_TURNED_IN")
frame:RegisterEvent("QUEST_ACCEPTED")