English or Anglo-American common law is often referred to in Spanish as Derecho anglosajón and, indeed, it is misleading to translate “common law” literally as Derecho común. In that regard it is important to note that in Spanish legal texts the expression Derecho común probably isn’t a reference to English or Anglo-American “common law,” but rather most likely denotes the “common law” of Spain, i.e., civil law, or the law of the Spanish Civil Code (Código Civil).
By virtue of their vecindad civil (“regional domicile”) Spaniards are subject to either Derecho (civil) común (the common law of the Civil Code) or Derecho (civil) foral o especial (local law). In that regard, an expression such as el presente contrato se rige por el Derecho común español implies that “the present contract is governed by the Spanish Civil Code.” In the context of sources of business law (fuentes de Derecho mercantil), transactions not covered by the Commercial Code (Código de Comercio) are likewise governed by Derecho común, once again being a reference to the law of the Civil Code: En ausencia de normas mercantiles se aplica el Derecho común, esto es, el Derecho civil. And an expression such as las CCAA de régimen común likewise refers to the “Autonomous Communities subject to civil law (or) the Civil Code, rather than to Derecho foral o especial (local law).
In summary, unless the context suggests otherwise, in Spanish legal texts Derecho común denotes Spain’s common law (the Civil Code). If not clearly a mistranslation, rendering Derecho común in this context as “common law” might prove to be a miscue, prompting readers to assume that the reference is to English or Anglo-American “common law.” Thus, in this case Derecho común is perhaps best rendered as “(Spanish) civil law” or the “law of the (Spanish) Civil Code.”
More here on vecindad civil and the regions in Spain that have their own local law (Derecho foral o especial).