Answering my own post: I found a way to do it. Probably not the official way, but it seems to have worked.
1. I fully exited Isotoxin (so its files were no longer in use).
2. I used 7-zip to open the isotoxin.data file in the program's directory (because that data file is really a zip file).
3. Within 7-zip, I navigated into the "loc" folder and deleted all the language-related files there except for the two for English.
4. I closed 7-zip. (Now the isotoxin.data file is smaller than it was.)
4. Now, when I run Isotoxin, all the features are in English.
I hope this post can help somebody in the future, as when I googled "Isotoxin" along with "English" (or "Language"), I got no useful hits, and this board won't let anybody search for "English" or "language" because it says those words are too common. Maybe that can be changed too?