Translation:I will buy a bag.
What about the kanji "鞄" ? which translates to the hiragana "かばん" ? Which means bag? This is the first time I've encountered just arbitrarily using katakana instead. When you say emphasis, you must mean written emphasis, so is that in the same manner as writing it in bold/italics for emphasis?
A moment of silence for those people who hastily entered "I bag a buy" and got it wrong... xD
Wouldn't this be wierd to say? Unless someone was asking me what I was doing at that exact second, but even then I'd be like 'カバンを買っていあす.' (I'm buying the bag.) When would I ever say 'I buy the bag.'? Is there some sort of Japanese social language context I'm missing? We don't even say this in English.
Don't forget that Japanese doesn't have a future tense, so this is also "I will buy a bag" if used with a time that's in the future. (午後七時にカバンを買います → I will buy a bag at 7 PM)
i can see it as being used as part of a narrative. something like:
"man, I don't need a bag. It's just a two day trip." I think as I look at the bags. Then I start listing everything I'm taking with me and realize that I'm a fool. I buy a bag.