Which programming or markup languages do you use most often with Textastic?

You can use Working Copy to manage git repos and open a repo from that within Textastic.

Erlang

Visual Basic .NET!!!

Fantom (fantom.org)

How about Basic languages, like Visual Basic? I don't see any syntax highlighting for them. It is good that we be able to assign a set of file extensions for one that syntax highlighting is available to have highlighting for them. For example, like notepad++, if I assign .bmx to visual basic kind set, notepad++ shows myfile.bmx with visual basic highlighting.

reStucturedText

added

Elm please

added

I also edit SVG (XML

I think that's covered by XML.

AsciiDoc, please

added

TypeScript? Does the existing JavaScript support already cover this? I see you do list CoffeeScript separately...

added

I would like to add MoonScript to the list.

.pug and .vue please.

How about HCL (HashiCorp Configuration Language), it's used by Terraform and GitHub Actions, to name a few. 

TOML please? Quite popular these days.

Added MoonScript, HCL, and TOML.

Twig

Go

Add KSP Kontakt, please, thou it’s very rare)

Please, it would be great if you could add IDL/ENVI, not so different to Python or Fortran code colors! :)

R Markdown

Hugo (GoHugo) 

https://gohugo.io

please, Visual Basic 

Added Go, Twig and Visual Basic.

How do you save/apply your selection? Each time I reload the list is empty

Try tapping/clicking on the name instead of on the check box. 

Worked!, you might want to mention that in the post, most people would click a check box and not the label. I only noticed the entry was ignored since I did a reload

This used to work, I'll open a bug report at UserEcho, where the forum is hosted. Thanks for letting me know!

It should now work when clicking on the checkbox. Thanks again for letting me know about this issue.