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Puerto Rico is the only current U.S. possession whose legal system operates primarily in a language other than American English: namely, Spanish. However, because the U.S. federal government operates primarily in English, the result is that all Puerto Rican attorneys must be bilingual in order to litigate in English in U.S. federal courts and to litigate federal preemption issues in Puerto Rican courts.
Puerto Rico had about a million residents at the time it became part of the United States, who ferociously resisted conversion to English before the U.S. government finally gave up in the 1940s. By way of contrast, the Spanish-speaking settlers in the vast territories obtained from Mexico after the Mexican-American War were promptly swamped by English-speaking American settlers, which is why the state governments that emerged in those territories all primarily use English today.
Many of the Laws of Puerto Rico (Leyes de Puerto Rico) are modeled on the Spanish Civil Code, which is part of the Law of Spain. After the U.S. government assumed control of Puerto Rico in 1901, it initiated legal reforms resulting in the adoption of codes of criminal law, criminal procedure, and civil procedure modeled after those then in effect in California. Although Puerto Rico has since followed the federal example of transferring criminal and civil procedure from statutory law to rules promulgated by the judiciary, several portions of its criminal law still reflect the influence of the California Penal Code.