Some comments based on about 40 different countries and dealing with designers, codes/standards and and contractors, but obviously a basic routine understanding of the metric systems and conversions to the inch-pound system is a given:
English/American is a very universal language is most developed countries because of the standards that were adopted as a part of the local codes and standards. In India, for example, the official language is Hindi, but is English/American for technical applications and it is used in all schools and codes/standards.
Because of the differences in the alphabet and sentence structure languages that are based on the Cyrillic alphabet can be difficult (Russian and Greek are examples). The oriental languages Chinese versions, Japanese, Korean and other Asian languages are also unique and require more time to be able generally read documents and speaking can be longer, although some newer systems developed in the last 20 years have sped up the time frame for speaking. Unless you are dedicated to working in that area, it is best to develop a basic understanding, but rely upon a local professional.
The European languages (Spanish/Portuguese, French, Italian, and German) are common in many countries because of historic immigration. Of those, German could be the best additional language, depending on what part of the globe you might concentrate on.
In much of Europe or areas of South Africa, German could be the best second language.
I have been startled by the ways languages are combined and used. I was on a site in Brazil where about 20 loadbearing masonry buildings (10 to 15 stories) using 6" loadbearing walls was under concurrently undergoing construction. After looking at the drawing that were a work of art in accurately and practicality for quality construction on a minimum number of sheets that were updated every two days and color coded to differentiate (and eliminate language problems) the block strengths (4 different block strengths color coded by strength in a general site inventory). I struggled when taking to the site superintendent that really spoke Spanish/Portuguese but understood some English/American. when the engineer showed up, I asked him what code the buildings were built according to, he replied "They are built according to you codes, but better and use TV to monitor representative core conditions to enforce a general standard. We learned from buildings in southern California and have used the more recent different American(ACI 530) updates to do it better". - They do it better than in the U.S.!
If you had to choose 1 second language to learn, it would depend on where you think or want to become involved. As a general tool, for technical, German would be good today, but the world in shrinking.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.