You should be able to open individual files from the iSH file provider using the "Open…" command in Textastic.

To be able to iSH folders as an external folder to Textastic, this feature would have to be added to iSH's file provider extension (similar to how this is implemented in the "Working Copy" and "Secure ShellFish" apps) - Textastic already supports this. 

This would be totally possible, but would require a major restructuring of the app so that the same file can be displayed and edited in multiple editors simultaneously. Currently each editor works on its own in-memory copy of the file. Maybe I'll implement that in the future.

I'm not sure if I understand correctly. Are those files stored on a server and you don't see them when connecting to the server with Textastic's SSH client?

Yes, that's a good idea. iPadOS 15 seems to add an official way to add tooltips using the new UIToolTipInteraction API and I plan to add it to Textastic along with other improvements for iOS 15.

Maybe there is a tweak installed that causes problems. I'm running out of other ideas.

Does changing to German do anything?

Are you running the latest iOS version? No jailbreak?

Even with these settings, Textastic shows in English for me:

I have duplicated your settings on my iPhone running iOS 14.6, but I cannot reproduce the issue. I see Textastic localized to English with these language settings.

Which iOS version are you running?

Can you try if tapping "Add Language…" and choosing English (UK) changes anything?

Can you show me a screenshot of Settings -> General -> Language & Region?

The preferred language order should have all languages you know.

This could be for example:

iPhone Language: "English (US)"

Preferred Language Order:

English (US)

German 

Maybe the list of preferred languages is missing English for you?