Translation:There are keys in the pocket.
So keys have a kanji, but are written in katakana? Is it a loanword from chinese?
Not really. If it were borrowed from Chinese, the on'yomi of 鍵, ken, would have been used. kagi is the kun'yomi.
"My keys are in my pocket" is not accepted, why? If "my pocket" is correct "my keys" should also be right...
There really is no indication of ownership in this sentence. Since あります pretty much just means there is/are/exists.. then the sentence is really just translating as "Inside the pocket, there are keys" (keys exist) etc.
This would make sense if say you found a jacket on the street with keys inside
Sure. It's not specified how many keys or which pocket, or how many pockets, or who those pockets belong to.
Japanese is very context dependent. So if you were talking about you jacket. Then said there are keys in thr pocket, it would be understood there are keys in your poccket. I think if Japanese like the way I make text messages. Since i hate typing on a small screen i dont use pronouns or articles. Like this "going to the store, getting milk". When my wife reads that she understands i mean "I am going to the store to get milk". So it is six words vs nine. The hardest part of japanese and also the easiest part is the simplicity
According to CaleGibbard above, ポケットに just means the pocket is the location, meaning it can mean inside the pocket, but it can also mean for example on the outside of the pocket. ポケットの中に unambiguously means inside the pocket.
No....In japanese nouns own their prepositional phrases with NO so that 'poketto no naka ni- means the pocket's insides