Why do you have to download all localizations? Isn't it possible to download only the required one after install? As far as I know Waze does this, for example.
(Honest question, I'm not a mobile developer)
Sure you can! Downloading the localizations is a large extra complexity in development though, while simply bundling them all is a minor bother in increased download size, for the customer.
because the complexity of development will increase many fold by doing so.