What Are Design Factors for Internationalization, L10n, Accessibility?

I'm working on designing Open Source software, and while I can't internationalize, localize, and accessiblize (!) it now, I want to make it easy to internationalize, localize, and accessibilize it later.

To design properly, I need to know the factors that go into those things. Please list all that you know of, whether for GUI's or TUI's. It's okay to repeat factors because it's good to know what's most important too.

Here's my non-complete list for each.

I18n:

* Be able to swap all output strings depending on language.

* If output needs to be formatted, be able to put the format items in any order because languages put things in different orders.

* Support for left-to-right, right-to-left, and boustrophedon [1] scripts. (Is boustrophedon necessary?)

* Support for vertical scripts, whether secondary left-to-right or right-to-left.

* Support for multi-codepoint characters. [2]

* Eliminating latin-1 assumptions. [3]

* Support for large character sizes, such as in Zalgo text. [4]

* Support for larger text than in the default language because some languages take more space than others.

L10n:

* Support for adjusting number formats.

* Support for adjusting date and time formats.

* Support for various keyboard layouts and input methods. [5]

* Support for different collation and sorting.

* Support for different currencies.

* Support for different measuring systems.

* Support for formatting phone numbers and addresses.

* Support for different ways of naming people, places, things, etc.

* Support for graphical adjustments, such as avoiding certain colors.

* Support for legal requirements. (This one is too open-ended, unfortunately.)

Accessibility:

* Support for text-to-speech.

* Support for easy keyboard-only navigation with text-based landmarks?

(Yeah, sorry for the small accessibility list. I'm not going to be a good accessibility developer, but I do plan to test my software blindfolded.)

[1]: https://en.wikipedia.org/wiki/Boustrophedon

[2]: https://manishearth.github.io/blog/2017/01/14/stop-ascribing-meaning-to-unicode-code-points/

[3]: https://manishearth.github.io/blog/2017/01/15/breaking-our-latin-1-assumptions/

[4]: https://en.wikipedia.org/wiki/Zalgo_text

[5]: https://en.wikipedia.org/wiki/Input_method