Custom scripts?

I do not think Readlang specifically supports any custom scripts or plugins (like, e.g. Anki).

However, it is possible to make adjustments using Tampermonkey scripts. See my post about it here:

So I had some success customizing Readlang with Tampermonkey scripts. The best part is that they work on mobile (with Edge browser).
I have two scripts.
One script adjust appearance of some parts of the interface (mostly font size). Basically, it just applies a custom CSS.
Another script translates words or phrases on select.
I use these scripts only on mobile, as desktop browsers support extensions that do the same but with better functionality.