Be prepared to be treated differently when working in another country than you would have been as a tourist. Ask others who have worked in this new location for advice before agreeing to start.
Confirm approaches, criteria and standards you've used previously apply before using them in a new environment. Sometimes, it's as simple as knowing what materials are commonly available in a new location. Also, know the local professional licensing requirements.
Most other countries use the metric system, now called the International System of Units (SI). If SI is new to you, you lose many of your sanity checks and have to create new cues to spot problems. Also, don't assume all you have to have are conversion tables to work in SI. After all, in the USA, even a 2x4 isn't two inches by four inches.
Be aware of cultural differences. Body language differences are often significant. A hand gesture that means one thing in one country may be insulting in another.
When using a translator, be aware you can't talk for a few minutes and expect the translator to remember everything you say. Say one or two short sentences before letting the translators do their thing. If your translator understands what you're doing technically, they may answer questions on your behalf, which means they will appear to be in charge and not you.
There are significant differences between an international assignment that is short-term, say a month, and one where you move and live in another country. In a long-term assignment, you should learn the local tax law or find you may owe money to another country you hadn't anticipated. Long-term stays also put stress on family units. And, by the way, don't think working internationally will resolve a dysfunctional family dynamic that already exists.
In some countries, ethical practices may not be up to ASCE's and your standard. Make an effort to ask others you trust how to handle ethical pressures without compromising your principles. Obviously, a country whose track record in individual rights is poor will be risky. For example, in one country, inspectors who complained to authorities that the contactor was doing poor work were beaten up, rather than listened to. In another, instance I still wonder if I had been used to smuggle electronics into a country.
The above points come from the book "The Engineering is Easy/Memoir of a Project Manager" which describes many issues I encountered while working outside the USA.
I had a three month internship in Austria right after finishing undergrad. What stuck to me the most about working in Europe is that, because everything is so geographically close, you are exposed to a variety of languages and cultures by traveling just a small distance. Even the smallish company I was working for had projects and collaborations all across Europe, Central Asia and in Saudi Arabia. The conference calls were especially fun. We had to wait for an Italian collaborator to come back to the office from his two-hour siesta, and had an Albanian consultant chime in with 'it will be stolen' in response to every suggestion to install this or that piece of equipment at a constructed wetland in his country.
I was also slightly surprised by all the stores in the small town closing promptly at 5 on weekdays and staying closed all day on Sundays, and by wine and beer in the company's lunch room.
Because it was a small town, I've literally met everyone and their grandmother. People took me to their farms, family wineries and camping trips. It was actually a little too much for an introvert like me. I am also aware of an opposite trap - if you end up in a place with many other expatriates, sometimes it's easier to hang out with them and never really get to know the people who live in the country.
From growing up in a country where small scale bribing is a way of life, It would be interesting to hear if anyone had experience with local corruption and how they handled it.