"Keen" is also commonly used in Australian English, and as others have said, it means to be interested in or attracted to something or someone. If you are keen on Jack, it means you have a crush on him, or are interested in him romantically. "Keen" can also mean "sharp" or "perceptive" in particular contexts, for example, you can have keen eyesight, a keen ear, a keen intellect. "Keen" as a verb can also mean to cry or wail in grief, but this is a rare use of the word.

To be 'keen' is used in British English. A similar adjective is to be 'eager'. Keen is often used to describe a strong interest in an activity. One would probably not use it with respect to going for McDonalds because it would imply the person goes to that restaurant frequently (unless of course that is exactly what you meant), but you may use it more frequently within the context of sports such as "She is a keen cyclist" or "He is a keen Arsenal football supporter", "They are keen sailors" etc. It may also be used within another context to indicate that someone is a little too eager. e.g. If after a first date you thought the person was trying too hard to make a good impression, you could described them as 'a bit too keen' to your friends to indicate that you were not too impressed with them... obviously it would be very bad etiquette to say that directly to the person in question as it would imply that you thought them to be desperate for your approval.

I am a Canadian native English speaker. We do not use the word keen, I don't think Americans use it a lot either. But, I have met many South Africans while traveling who use "keen" a lot, which makes me think it could be British. But yes, as others have commented, "keen on" means "interested". Here are some examples. A: Are you keen on McDonald's for dinner? B: sure, I'm keen. A: Are you interested in McDonalds for dinner? B: Sure, I'm interested A: Do you want Mcdonalds for dinner? B: Sure, I'm keen.

I think the phrasal verb of "keen on" is similar meaning and usage of the phrasal verb of "interested in"