Which language are you interested in adding to the keyboard? The best way is by requesting it via Feedback Assistant.

According to the Bidirectional Algorithm (http://www.unicode.org/reports/tr9/), for the text “10'20"”, when it is presented in a text element whose directionality is right-to-left or natural (with the UI Language set to Arabic/Hebrew), the two elements “10'” and “20"” will change visual positions compared to a left-to-right context, i.e. it will be what you say you’re seeing on iMac. However, depending on what this text semantically conveys, the Bidirectional Algorithm can and should be overridden with markup with the use of characters like U+200E LEFT-TO-RIGHT MARK, U+200F RIGHT-TO-LEFT MARK and many others. You will notice that the outputs from many of the Formatter APIs include this markup to ensure that data is presented correctly. To answer the question of why you’re specifically seeing a difference, we’ll need more details. I would suggest creating a sample project (or 2 projects, one for each platform) and filing a bug report via feedbackassistant.apple.com and mentioning the FB# here.

The list of keyboard layouts supported on iOS & macOS is always changing/expanding. Additionally, users can install their own keyboard layouts in ~/Library/Keyboard Layouts. Can you share more details about exactly what kind of management tool you’re building and why needing a list of all layouts would be a requirement for it?

I have had no success in using Shanghai romanization or Sichuanese pinyin This mechanism is still based on Mandarin Pinyin (汉语拼音) and what you should expect to see is that Cantonese, Shanghainese or Sichuanese phrases typed via Mandarin Pinyin will produce more accurate suggestions based on the chosen Regional Dialect. For any enhancements that you’d like to see, please use http://feedbackassistant.apple.com. Thank you!