A colleague has actually written on similar issues - this paper may be of interest? Śliwa, M. and Johansson, M. (2014) How non-native English speaking staff are evaluated in linguistically diverse organizations: A sociolinguistic perspective. Journal of International Business Studies, 45(9): 1133-1151.

I know an eminent mathemartician, who speaks Russian, but not English! I think there is a psychological barrier for that. Another factor cold be the national identity.

Parviz: Here is a study on the causes of common stumbling blocks in international communication situations, and practical remedial action. The author is a voice and speech specialist, and she focuses on speakers with good command of English as a further language who are interested in engaging their audience more effectively and improving their intelligibility:

Maggor, R. (2011). Empowering International Speakers: An Approach to Clear and Dynamic Communication in English. Voice and Speech Review, 7(1), 178-187: http://www.tandfonline.com/doi/abs/10.1080/23268263.2011.10739539#.VN3ZxsZQJGR

Madalena

Agreeing Siir Costas's answer. But Sometimes subjectivity is also there with priorities..

Intelligent, high performing individuals fear embarrassment and feeling inadequate. It is necessary to practice a language with native speakers. There are many professionals who speak a foreign language well with others from their native country who learned and practiced only in their native country.

I agree with Joseph. High expectations may hinder performance in talented individuals and contribute to language anxiety. But I would also think that the target language in question is another factor. I think that there is an expectation in academic circles that professionals have to be proficient in English, while additional languages are seen as 'added bonuses', but not essential tools. Thus the level of stress related to performing in English is higher than when dealing with other languages.  

Susana

True. At least it contributes to my anxiety in English. (My native tongue is southern US English.) With most US citizens the problem is other languages and not having an easy opportunity to practice another language.

Hi Parviz

Modularity theory , compartmentalized , Non-unitary competence could expound the asymmetrical development, imbalanced performance and nonproportional cognitive competency as well as uneven cerebral dominance manifested in even  highly talented  individuals,

Conversely, just take  into account  savants ,  who show impressive massive mental capacities or abilities but may be suffering from  neurodevelopmental disorders like autism. They demonstrate extraordinary  skills in certain areas like calculation , poetry, and art.