The problem of English is not so much colloquialisms or slang.
It's a history of being conquered over and over and mixing the various languages together, throwing in a major vowel shift and then some scholars decided to further change the spelling of some words, just because.

Let me just say, I'm not necessarily blaming anyone for the mess that is English.
I merely point out that there is often no clearly recognizable correlation between spelling and pronunciation, which can easily trip up non-native speakers (ant often natives too).