What are the best practices for teaching second-language students in classes that have moved online mid-semester due to COVID-19?
how can professors teach their online classes in a way that is easily accessible for students whose first language is not English?


Teaching online does not change the students' brains.  Use all the pedagogical techniques you would use in a lecture or discussion class that was not online.  They will still work.

One thing that can be easier online:  you can record your lectures.  This can be done for in-person classes, but typically online teaching software makes it easy.  Lecture recording allows English language learners to listen repeatedly, at their own pace.  Just keep in mind that some students need privacy to practice speaking.