If a native English speaking person on trial decides to exercise his right of freedom of speech by speaking in a foreign language (i.e. Chinese), does the court have to hire a translator?
Not necessarily. The right to a translator is derived from the 5th, 6th and 14th Amendments, and the prohibition against discrimination based on national origin (the Civil Rights Act). However, SCOTUS noted in Perovich v. US, 205 US 86 that appointment of a translator is a discretionary matter for the court (this was simply mentioned, without details as to why a translator was asked for). There is a federal law (applicable to federal cases) that sets up an infrastructure for providing interpreters, but this does not create a right to an interpreter. In US ex rel. Negron v. New York, 434 F. 2d 386, defendant was not afforded a translator throughout the trial, and the court found that
The least we can require is that a court, put on notice of a defendant's severe language difficulty, make unmistakably clear to him that he has a right to have a competent translator assist him, at state expense if need be, throughout his trial
The question which the court has to ask and answer is whether the defendant is capable of competently assisting in his defense were the trial conducted in English. If a person is perfectly bilingual in English and Mien but decides to testify in Mien to be annoying, the court will not pander to the defendant's desire to be annoying. If the person has a weak grip on English and can more effectively testify in Mien, then (after cogent arguments to that effect) the defendant should be provided a translator. The question that the court has to address is necessity.