Frankie by Creativecrap

Frankie (formerly OkidoKey) includes an in-app keyboard with a plain text editor and keyboard extension for system-wide usage. And auto-copy Clipboard editor for typing without creating or opening a document.
Multiple input methods in one keyboard extension, typing English without switching to another keyboard, supports emoji, lexicon, spell-checks, predictions, and smart punctuation to improve quick typing.
- Includes most common and ready to use Chinese input methods and keyboards, i.e. Array, Cangjie, Dayi, Pinyin, etc.
- Capable of using custom input methods
- Capable of using multiple input methods and keyboards
- Sibling Keyboard and Key Flicks for Half/Full-Width symbols (iPad)
- Composition prompt option
- Navigation Keyboard
- Bluetooth keyboard supports (in-app text editor only)
- Auto-copy content Clipboard editor
- Chinese and English mix-typing (trigger by return key)
- Chinese lexicon and Emoji
- Smart punctuation
- Homophones and Phonetics Lookup
- Traditional and Simplified Chinese output
Input Methods
- Optimized for Array input method and fully support for special and shortcode mode
- Zhuyin input method supports the standard, Et26, Et41, Hsu and IBM keyboard layout
- To install custom input methods like Boshiamy by selecting the CIN table file from the file browser and choose install
External Keyboard
External keyboards are not supported by keyboard extension, however, you can use Clip Pad in split-view as a bridge for inputs, with auto-copy Clipboard all you need is switching to the next app then tap paste icon or cmd-v to paste the content.