If they're only using React Native for 3 or 4 features (as they say in the article) I could see how adding in i18n support would be challenging as you'd now have to maintain Java/Kotlin, ObjC/Swift, JS versions of this. But thats just true of the whole approach in general. I think if you're using RN for only 20% or less of your product, it probably is more trouble than it's worth.