Have you ever picked up a pattern and started to knit or crochet your swatch (test piece to check tension) and decided that the pattern didn’t look like the picture? That might be because you have misread the pattern or it might simply be that the pattern, although written in the English language, may have been produced in one of any number of English speaking countries. As with general language, although the main body of vocabulary is the same, there are many differences in the meaning of some of the words. For instance the part of a car that lifts to expose the engine is called a bonnet in the UK whereas in the US it is called a hood.
The same applies to both knitting and crochet terms, also references to types of yarns, needle and hook sizes. This can be very confusing.
Most modern patterns these days will reference both the local needle or hook size and the metric sizing. This helps a lot with cross-referencing but if you have been collecting free vintage patterns from the internet, then many of these will not have the metric sizing to help you.
You should be able to find most of the conversions you need here but if there is something you think would be useful to add please feel free to request it on the chat page comments.