UDRP rules require the proceeding to be conducted in the language of the registration agreement.
In circumstances where the website is in English, the host is in the US, the registrant is in the US, etc., panels regularly permit proceedings in English, regardless of the language of the registration agreement.
http://www.wipo.int/amc/en/domains/search/overview/index.html#43
4.3 What is the proper language of the proceeding?
Consensus view: The language of the proceeding is the language of the registration agreement, unless both parties agree otherwise.
Relevant decisions:
Telstra Corporation Limited v. Telsra com /Telecomunicaciones Serafin Rodriguez y Asociados D2003-0247 < telsra.com>, Transfer
Fondation Le Corbusier v. Monsieur Bernard Weber, Madame Heidi Weber D2003-0251 among others, Denied, Transfer in part
Advanced Magazine Publishers Inc. v. Computer Dazhong D2003-0668 , Transfer
However: In certain situations, where the respondent can clearly understand the language of the complaint, and the complainant would be disadvantaged by being forced to translate, the language of proceedings can remain the language of the complaint, even if it is different to the language of the registration agreement.
Relevant decisions:
LโOreal S.A. v. Munhyunja D2003-0585 , Transfer
Deutsche Messe AG v. Kim Hyungho D2003-0679 , Transfer