My biggest complaint about the Readlang was the lack of ability to easily adjust the font size in the Explain texts on mobile. So, I started looking for methods to do it outside Readlang.
First of all, I tried to use font size settings on several mobile browsers (on Android). That works to a degree, but if I set zoom more than 125%, the interface becomes distorted too much and unusable.
I searched for other methods, tried to use bookmarklets, but they did not work on Chrome in Android.
I tried to use Kiwi Browser, because it supports text reflow on zoom, but this method does not work on Readlang. Then I learned that Kiwi Browser supports extensions. I was able to install Stylus and create a style for Readlang. Finally, it worked as needed. However, Kiwi Browser lacked in other functionality and was quite buggy. And apparently it is obsolete now. Browser creators recommend to use Edge.
Edge on Android indeed supports a limited range of extensions now. Stylus was not among them, so I was frustrated at first. Then I decided to check carefully the entire list. While Edge does not support Stylus, it does support Tampermonkey! So, it is possible to use Tampermonkey script to apply the desired formatting where needed. It is less convenient than Stylus, but it works. That is the solution that I am using now.
Still, it would be nice to have the ability to adjust Explain font size independent of the browser.