My own first response is "you can't." Except possibly by factory reset which is a pain. I got no joy from chat with an Amazon "expert." The problem: In the last few days whenever I highlight a word on the Kindle and connect to Wikipedia, suddenly I'm getting an Arabic version. My device default language is English. The default dictionary I've selected is English. I have never connected to an Arabic version of Wikipedia. I have no books (out of hundreds) that are in Arabic. Some time ago I downloaded two different Arabic to English dictionaries, but these are not the default dictionaries, and I have removed them and restarted the device and this has naturally had no effect. I have tried manually getting to the English version and while this works, it does not reset the default connection to the Arabic site when looking up a word from inside a book. There are no settings to control this on the Kindle; I haven't found any answers on internet search or on Wikipedia itself but I don't quite know how to ask the question. If you cannot answer this but can suggest a better way to research this, please let me know!
Further evidence regarding this problem:
I have found that it is with one particular book that I have this issue. Nothing indicates why it should have happened; it's a kindle edition of a older British book (P.G. Wodehouse). I made the discovery when I tried to change the font and discovered that unlike other kindle format ebooks I only had a choice to change the font size, not the font; and the examples were in Arabic. No clue why this should be so. At this point I tested whether I still had the ability to change fonts in other books (I do) and then tested the connection to Wikipedia, which was the default English version. Does that make the version of this book somehow pirated or otherwise undesirable?