An ever so slighlty off-topic question to see on a biology site, trust me I know:)
Recently, a University Professor came in and gave the school a presentation on the importance of being able to speak two languages in ANY career. Do employers in the field of science (mainly palaeontology, evoultionary biology or astronomy) even consider bilinguality when choosing employees? and, if so, from your experience, which language would be more useful in these fields, French or Spanish?

Also, the Professor said that when an employer is looking at two different possible employees, if one is bilingual and the other is not, he is almost SURE to pick the bilingual option. Is this even true at at all, or is this more of a Professor trying to popularise his field of work?

At this point English is pretty well established as the international language of science, and universities and other organisations that might employ scientists are likely to be a lot more interested in things like publications, grants and teaching experience than in an applicant's proficiency in languages other than English. So yes, I'd say that the professor who addressed your school was exaggerating a bit if he said that foreign language proficiency was important in ANY career. Of course, it's (unfortunately) more or less essential for a scientist whose mother tongue is not English to develop the ability to publish and present his or her work in English in order to have an international impact.

With that said, proficiency in at least one foreign language is useful even for native English speakers like myself. Scientists need to travel internationally and communicate with foreign colleagues, and facility with languages certainly makes that easier. I live and work in China, for example, and I find myself constantly wishing my Chinese were better than rudimentary. Also, a fair amount of older scientific literature is in languages other than English, although this is more of an issue in some disciplines than others.

Whether French or Spanish is the better choice will depend to some extent on an individual scientist's activities. A palaeontologist who visits France often, collaborates closely with French colleagues and does fieldwork with them in Morocco every summer will probably want to learn some French. However, Spanish has many more native speakers worldwide and is the primary language in many more countries. Important scientific literature has been, and to some extent continues to be, published in both languages. Both are undoubtedly useful, but if you're in school and need to make a choice I would suggest that you opt for Spanish unless you have some specific reason (like thinking you might want to work in a part of the world where French is widely spoken) for choosing French instead.

In all my time as research team leader I have never once taken into account whether a person can speak more than one language when hiring research staff. I focus on the quality of their research hands on experience and skill set, whether they have developed and written grants and papers and whether they seem collaborative and pleasant to work with. As long as they speak English well enough to communicate with me and the rest of the team and write good scientific English then my interest in what other languages they can speak is minimal.

I agree with the above comments. Rightly or wrongly English is the de facto language of science so for non-native speakers it is essential to learn if you want to have an international profile. 

Of course if English is your first language then learning a second can certainly open up more options for you in an academic career. For instance if you wanted to work in a French university you would likely be expected to teach undergraduates in French even if your research group worked in English. Similarly in some universities (e.g. in Quebec) teaching is done in two languages. In such cases being bilingual is obviously a big asset and could set you apart from other job applicants!

French or Spanish? I'm not sure one is more useful for science than the other so I wouldn't consider this a factor in deciding what to study.