For iPhones why not try SwiftKey?
SwiftKey Keyboard uses Artificial Intelligence to automatically learn your writing style, including the emoji you love to use, the words that matter to you and how you like to type. That means autocorrect and predictive text that actually works because it adapts to you. Swiftkey Keyboard is available in 80+ languages, including Welsh.
Why not use Swiftkey to write Welsh Twitter or Facebook posts?