スーパーに買い物に行ったらお相撲さんが居たって英語でなんて言うの？

The most common way to explain that you saw something/someone at a specific place is to say "I saw (something/someone) at a/the (location)." It's the shortest and most concise way to explain what you saw and where you saw it. Or, in the second example, you could state where you went to first and then what you saw: "When I went to the (location), I saw a (person/thing) there." This expression is also used, but it is more wordy.

"I saw a sumo wrestler at the supermarket." - In this sentence you are telling someone what you saw, the subject being the 'sumo wrestler' and then also going further to share the location where the subject has been seen, namely the supermarket. "At the supermarket, I saw a sumo wrestler. " - This sentence conveys the same meaning as the first, the sentence order is just different.

Usually when sharing information like this there is a reason. You may add an emotion or adjective regarding why you are sharing the information. You could say I was happy to see a sumo wrestler at the supermarket. You could add an adjective to describe the sumo wrestler to your sentence as well, "I went to the super market and saw a good looking sumo wrestler". Or you can just clearly state the obvious, "There was a sumo wrestler at the supermarket".

The above sentences mean the same thing but structured differently. Example: A:When I was at the supermarket, I saw a sumo wrestler. B:Oh wow,lucky you. Did you get an autograph

You may just state what it is you saw and where: "I saw a train going through a tunnel." You may add as much detail as you like: ""I saw a train going through a tunnel, last Friday." Or you could make your story more captivating by playing "Guess who?"

When you want to say you saw something somewhere then the simpiliest way to say that is 'I saw a............................ when I went to ............................' another way to say this is 'when I went to the supermarket I saw a sumo wrestler'

"While I was shopping at the supermarket, I saw a sumo wrestler." While - during the time you were at the supermarket. I saw - something that you personally saw. Not something you heard about, but something you saw. You could also say... "I was surprised to see a sumo wrestler in the supermarket, today." - In this sentence, you express your surprise to have seen a sumo wrestler. "Did you see the sumo wrestler in the produce section?" - Here you are asking someone else in the store if they saw the sumo wrestler.