Where I live (Philippines) you have to shower at least once a day. Super easy to get BO in the tropics.

If that person of a different nationality is warm/friendly, it's a-okay to greet them in their native language. I'm sure a lot of people would find it charming. Kamusta?

Before I got my last job, I emailed the HR manager once every 2 weeks or so. It helps you come off like you want the job but not desperately. Make sure to always reply to the latest message in the trail. Don't worry if they don't contact you right away; took me a month before I nabbed the appointment. For the interview itself, best thing to do is practice answering questions, even just once, with yourself or a friend. It's the same shit every time anyway. Lifehacker is a godsend.

Random adult things:

Keep your room tidy. Ditto for your car. That's how you get laid.

Own up to your mistakes.

If you split up with your SO, don't contact them for at least a year. Use that time to regroup and heal. Unless you have kids or property together or something. But you don't need to be friends. Focus on becoming a better you.

One last thing, no adult ever figures it out 100%. Just keep giving a shit, avoid getting complacent, and you'll be alright.