These similar expressions reflect two very different legal concepts. En légitima defensa is the Spanish criminal law expression for “in self defense” and legítima defensa may contstitute a circumstance that exonerates one from criminal liability (eximente de la responsabilidad criminal). In that regard a person repelling an unlawful aggression (agresión ilegítima) in self-defense or in defense of his rights or the rights of others (en legítima defensa de la persona o de derechos propios o ajenos) may in certain circumstances be deemed free of criminal liability.
In contrast, in procedural terminology (derecho a la) autodefensa is a totally unrelated expresion denoting a party’s legally-established right (in limited cases) to represent himself at trial without the assistance of a lawyer or (in Spain) a procurador (litigar sin abogado o procurador). In the US this is known as “pro se (or) pro per representation,”* and in this context autodefensa denotes “pro se (or) pro per defense,” while el derecho de asumir (or) ejercer la autodefensa is expressed as the “right to proceed pro se (or) pro per.” In England and Wales “pro se litigation” is called “litigation in person,” and a “pro se (or) pro per litigant” is a “litigant in person.” “Self-represented litigant” and “unrepresented litigant” are additional variants.
And the idea that representing oneself in court may not be a good idea exists in both languages. Indeed, the well-known admonishment “he who is his own lawyer has a fool for a client” is often reflected in Spanish as quien se defiende a sí mismo tiene un tonto por cliente.
*pro se, Latin for “for oneself” or “on one’s own behalf;” pro per, a shortened form of the Latin expression in propria persona