This is inspired by this question. Assume that a person has been subpoenaed to testify in a case (either civil or criminal – the person is not the defendant or a party in the case), and the person speaks English fluently, plus some other language "well enough" – let us say that he speaks Tigrinya. The person decides that he wants to testify in Tigrinya, so he gives his testimony in Tigrinya. He does not demand that questions be posed in Tigrinya, he simply opts to respond in Tigrinya for "artistic" reasons. Is there a statutory or case law basis in the US on which the court can compel him to testify in English? Various provisions whereby people who don't speak English have a right to an interpreter don't apply, since he does speak English. Tigrinya court interpreters are common enough that this would not impose an overwhelming burden on the court. Does the answer change if the language is Karuk, for which interpreters are unavailable? I am specifically interested in the substantive basis for the conclusion.
Can you be compelled to testify in English?
Yes insofar as the witness's reasons for not testifying in English are merely "artistic".
Courts allegedly procure efficiency, economy, and ease of access. Translation costs and the possibility of translation inaccuracies (or perhaps witness's pretenses thereof) contravene these criteria. The witness's fluency in English makes it highly doubtful that allowing him to testify in a foreign language would improve at all the ease of access to evidence. Furthermore, the witness's "artistic reasons" are very likely to be perceived as vexatious.
Duha v. Agrium, Inc., 448 F.3d 867, 876 (2006) points out that "[c]ases involving foreign language documents normally consider their impact and perhaps the cost of translation in the balance of convenience" (citing cases). Although this issue generally is a factor for deciding whether a U.S. court is forum non conveniens, various court decisions serve as guidance on what approach to adopt on your question.
For instance, Re Nine West Holdings, Inc., (U.S. Bankruptcy Court, Apr. 2020) makes the remark that "communications between the parties were conducted in English", whence there seems to be no actual impediment for testimony in English. Also the "inconvenien[ce] to either party to have to litigate [...] in a foreign language", Id., implicitly weakens the witness's "artistic reasons" for testifying in the U.S. in that foreign language. Conversely, see Takiguchi v. MRI International, Inc., (U.S. Dist. Court, Nevada, 2015), where a majority of evidence being in a foreign language "weighs in favor of dismissal". See also *Interface Partners Intern, LTD. v. Hananel, 575 F.3d 97, n.12 (2009).
These decisions suggest that a court would rather compel testimony in a form that is readily available. In the context of your question, this would mean testimony where witness's fluency would render a translator's involvement redundant. Regardless of the availability of translators of a foreign language in particular, a witness would need a good reason to testify in that language, lest he be considered vexatious.