Hello,
Thank you for your review. As this was a translation I did more than a decade ago, when I was a young and naive student, I cannot recall the exact reason why I decided to use the preposition "con" instead of "en". In any case, even in English "with this shirt" would express the same basic idea. I cannot imagine why I specifically decided that "en esta camisa" wouldn't work, but I was certainly wary of changing a preposition for its "direct" equivalent given that this often doesn't work (e.g. in the morning → por la mañana, in a hurry → de/con prisa).
Having said that, my attempt to be more liberal in the translation was no doubt a little overzealous on my part and I'm sure that, as a native Spanish speaker, you have a better insight into what works best in Spanish (which, for me at least, is more important than trying to find direct one-to-one equivalents like "in → en" and "con → with").
If you would like to make some more specific suggestions ("really bad" is a little vague), I will happily update my translation. However, at this point, it might be better off starting a new one from scratch, since jumped-up British students are not typically the best English → Spanish translators in the first place.
Thanks again!