37 C.F.R. 1.69 Foreign language oaths and declarations.

1. (a) Whenever an individual making an oath or declaration cannot understand English, the oath or declaration must be in a language that such individual can understand and shall state that such individual understands the content of any documents to which the oath or declaration relates.
2. (b) Unless the text of any oath or declaration in a language other than English is in a form provided by the Patent and Trademark Office or in accordance with PCT Rule 4.17(iv), it must be accompanied by an English translation together with a statement that the translation is accurate, except that in the case of an oath or declaration filed under § 1.63, the translation may be filed in the Office no later than two months from the date applicant is notified to file the translation.

37 CFR 1.69 requires that oaths and declarations be in a language which is understood by the individual making the oath or declaration, i.e., a language which the individual comprehends. If the individual comprehends the English language, he or she should preferably use it. If the individual cannot comprehend the English language, any oath or declaration must be in a language which the individual can comprehend. If an individual uses a language other than English for an oath or declaration, the oath or declaration must include a statement that the individual understands the content of any documents to which the oath or declaration relates. If the documents are in a language the individual cannot comprehend, the documents may be explained to him or her so that he or she is able to understand them.

The Office will accept a single non-English language oath or declaration where there are joint inventors, of which only some understand English but all understand the non-English language of the oath or declaration.