You should hear the Japanese commands as spoken by people with British regional accents, we've had Japanese nationals totally bemused before now.
I have to say, if I were to pick two combinations resulting in "the most humorous mispronunciation of a foreign word in one language by a native speaker of the another," it would be British English -> Japanese, hands down. I generally try not to laugh or judge, because it's not to be expected that one would know how to pronounce a word in another language, and any attempt is commendable, but I can't help but chuckle when I hear a British accent applied to Japanese words, if I can make out the word to begin with.
Now, the strange/difficult thing is to hear someone speaking who is in the process of learning the language, and far enough along to have a decent vocabulary and speak coherently, but who still retains a very strong accent. We've all experienced this in our native language, but experiencing it in your second or third language is interesting; I've heard people speaking Japanese with a very strong American, British, Chinese, German, and Spanish accents, among others. The strange thing is that, when it's strong, it can be very hard to understand, even if you're familiar with that accent (ie, American, or British), if you mostly only listen to native speakers. It also brings up some interesting questions about accents, and why some people retain them and others don't. Some people quickly adopt a very natural, fairly native accent by the time they reach a basic level of fluency, whereas others retain a thick accent even after living in a country for many years, and gaining a similar level of proficiency. I guess some people are more just more or less fluid in reforming certain habits?
That ability to recognize, and readily reform habits is, in my opinion, a very important skill in language learning as well as martial arts.