iPhone stock keyboard is horrible for foreign languages, and just flat out doesn't offer auto suggest for a lot of them. And since their third party keyboard API is still very buggy after two major iOS releases you really don't have any good alternatives.
My experience is mostly focused on SE Asia but nobody here sends an SMS if they can help it at all. They're fairly expensive, especially given that monthly 3g plans are as cheap as $3. Whatsapp, Facebook and Viber own messaging here.
I have literally never seen anyone using anything but the stock keyboard in iOS, and I live in Thailand, surrounded by locals who more often than not don't speak English.
Line is popular here, but only because it has stickers and iMessage doesn't.
> iPhone stock keyboard is horrible for foreign languages, and just flat out doesn't offer auto suggest for a lot of them.
That's not true. Why do you say that? I've used the foreign keyboards for Spanish and for Korean, and both keyboards are excellent and offer good autocorrect. I've also never heard anyone complain about the foreign keyboards being bad before.
I'm stunned at the number of people who use Apple Maps in Singapore. Google Maps is much better here - but about 50-70% of the people I know who have an iPhone just use the Map App that is installed on it. Kills me.
I don't know any english speakers with an iPhone who have changed out their keyboard - I've played with the others, but perhaps it's a foreign language thing?