Laudator Temporis Acti

Cassius Dio 60.17.4 (tr. Earnest Cary):

During the investigation of this affair, which was conducted in the senate, he [Claudius] put a question in Latin to one of the envoys who had originally been a Lycian, but had been made a Roman citizen; and when the man failed to understand what was said, he took away his citizenship, saying that it was not proper for a man to be a Roman who had no knowledge of the Romans' language.

Similarly, in the United States, one is supposed to know English before becoming a naturalized citizen:

(a) No person except as otherwise provided in this subchapter shall hereafter be naturalized as a citizen of the United States upon his own application who cannot demonstrate—

(1) an understanding of the English language, including an ability to read, write, and speak words in ordinary usage in the English language: Provided, That the requirements of this paragraph relating to ability to read and write shall be met if the applicant can read or write simple words and phrases to the end that a reasonable test of his literacy shall be made and that no extraordinary or unreasonable condition shall be imposed upon the applicant....

8 U.S.C. § 1423 (a)(1). So far as I can tell, there is no obligation for citizens by birth to learn English.

But a citizen ignorant of English is barred from at least one civic activity — knowledge of English is a qualification for sitting on a United States District Court jury:

(a) The chief judge of the district court, or such other district court judge as the plan may provide, on his initiative or upon recommendation of the clerk or jury commission, or the clerk under supervision of the court if the court's jury selection plan so authorizes, shall determine solely on the basis of information provided on the juror qualification form and other competent evidence whether a person is unqualified for, or exempt, or to be excused from jury service. The clerk shall enter such determination in the space provided on the juror qualification form and in any alphabetical list of names drawn from the master jury wheel. If a person did not appear in response to a summons, such fact shall be noted on said list.

(b) In making such determination the chief judge of the district court, or such other district court judge as the plan may provide, or the clerk if the court's jury selection plan so provides, shall deem any person qualified to serve on grand and petit juries in the district court unless he—

(1) is not a citizen of the United States eighteen years old who has resided for a period of one year within the judicial district;
(2) is unable to read, write, and understand the English language with a degree of proficiency sufficient to fill out satisfactorily the juror qualification form;
(3) is unable to speak the English language....

28 U.S.C. § 1865 (a) & (b). Some states have a similar requirement, e.g. Minnesota:

(a) The jury commissioner shall determine on the basis of information provided on the juror qualification questionnaire, supplemented if necessary, whether the prospective juror is qualified for jury service. This determination shall be entered on the questionnaire or other record designated by the court.

(b) To be qualified to serve as a juror, the prospective juror must be:

(1) A citizen of the United States.
(2) At least 18 years old.
(3) A resident of the county.
(4) Able to communicate in the English language....

Minn. R. Gen. Prac. 808.