Previous research has shown strong evidence that there is a language bias in academic databases in the social sciences and humanities[1]. However, evidence for the sciences is mixed: Some cannot find an US bias using the Science Citation Index (SCI)[2], others actually identify also a strong language bias in the SCI[3].
Furthermore, you have to consider differences in publication cultures between disciplines: The humanities and social sciences still approve monographs and edited books as publications which are seldom listed in theses databases.[4]
References:
[1] Bookstein, A./Yitzhaki, M. (1999): Own-language preference: A new measure of “relative language self-citation”, in: Scientometrics, 46 (2), 337-348.
Hicks, D. (1999): The difficulty of achieving full coverage of international social science literature and the bibliometric consequences, in: Scientometrics (1999), 44 (2), 193-215.
Hicks, D./Wang, J. (2011): Coverage and overlap of the new social sciences and humanities journal lists, in: Journal of the American Society for Information Science and Technology, 62 (2), 284–294
Yitzhaki, M. (1998): The ‘language preference’ in sociology: Measures of ‘language self-citation’, ‘relative own-language preference indicator’, and ‘mutual use of languages’, in: Scientometrics, 41 (1-2), 243-254.
[2] Van Leeuwen, T. N. et al. (2001): Language biases in the coverage of the Science Citation Index and its consequencesfor international comparisons of national research performance, in: Scientometrics, 51 (1), 335-346.
[3] Luwel, M. (1999): Is the science citation index US-biased? in: Scientometrics, 46 (3), 549-562.
King, D. A. (2004): The scientific impact of nations, in: Nature, 430, 311-316.
[4] Nederhof, A. J. (2006): Bibliometric monitoring of research performance in the Social Sciences and the Humanities: A Review, in: Scientometrics, 66 (1), 81-100.