In 1 week I will be travelling abroad for 2 weeks due to work, something that I did not mention at my interview.
This is something you definitely do want to mention during an interview. The fact that you didn't isn't great. I wouldn't disqualify a candidate over it but it's definitely a black mark on your application. You'll want to make every effort to be professional and easily reachable from now on to mitigate concerns they may have over your attention to detail, planning and communication.
Would it be considered poor of me to write an email now and inform them about my travel plans (and at the same time ask when/if they have decided)?
Ordinarily you wouldn't reach out until a week after the deadline they gave you, which means end of next week. In your case that's simply not an option so I'd reach out
early next week to let them know that you'll be out of the country. (Edit: Since you clarified that your travel is already starting next week, it's better to reach out immediately so they have time to contact you.) Be apologetic that you forgot to mention it and express interest in continuing the process. Detail the ways in which they can still contact you, if any, which will depend on the nature of the travel. Keep the focus on the fact that you'll be difficult to reach. You don't want to ask for an update unless you can do so tactfully as you want to avoid looking like you're hounding them.
If they're like the majority of companies when it comes to hiring this won't be a huge obstacle. But if they're in a rush to fill the position they may move on with other candidates so you should be prepared to be dropped from consideration. Government positions in particular often consider this a deal-breaker as they're required to adhere to strict (i.e. nonsensical) interview policies.