So, I have an old box of Koopman Oud-Hollandse Appelkaneel cake mix, which lists on its ingredients "Raising Agents 450 and 500", and looking online, it appears that Raising Agent 500 is baking soda, from some online research. The recipe for baking it requires you to add 1 large or 1.5 diced apples to the cake mix. I was thinking about replacing this with a roughly similar amount of frozen mango chunks instead, since I have then readily on-hand. Would this be a workable substitution? Would the acidity of the mangoes suffice to activate the baking soda, if that was part of what the apples were for? Would the fact that the mangoes are frozen while the apples called for in the recipe affect things? 
You probably don't need to worry, but I'd add a little lemon juice anyway. E450 reacts with baking soda (technically E450 can describe a few related  compounds, of which only some are used as raising agents. So the raising isn't wholly reliant on acidity from the fruit.  However it's possible that there isn't enough pyrophosphate to fully react with the baking soda. You need around 3× as E450 much compared to E500 by mass to complete the reaction depending which exact compositions are used, and all you know from the ingredients list sequence is that there's more E450 than E500. You're right that mango isn't as acidic as apple and a large apple (assumed to be  150g) would have very roughly 1.5g of malic acid, which we might want to replace Within the accuracy we've got here, malic acid and citric acid require the same mass to react completely with baking soda. A teaspoon of lemon juice has about 0.25g of citric acid, but I'm not going to suggest you add 6tsp of lemon juice to the mix, partly because mango definitely contains some acid (mostly malic)of its own.  Unfortunately I don't have access to the right journal papers to find out how much; anyway it reduces as the mango ripens and you don't know how ripe your frozen mango is.  Besides, chopped apple won't release anywhere near all of it acid to the mix. I suggest that it would do no harm to add a teaspoon or two of lemon juice.  It might even improve the flavour as mango tends to be sweeter than apple meaning the cake could end up too sweet otherwise. 