Comisión may certainly be translated as “commission” in many legal contexts. A primary example is Spain’s Comisión Nacional del Mercado de Valores, which following the model of the US Securities and Exchange Commission may be appropriately rendered as the (Spanish) “National Securities Market Commission.”
But rather than “commission,” comisión often refers to a “committee” and must be translated as such. This is particularly true in corporate law contexts where much of the work of the boards of directors (consejos de administración) of large Spanish corporations is often handled in a series of committees including, among others, a Comisión Ejecutiva (“executive committee”), Comisión de Auditoría (“audit committee”), Comisión de Nombramientos y Retribuciones (“appointments and remuneration [or] compensation committee”), Comisión de Buen Gobierno (“corporate governance committee”), or some combination of the above.
Likewise, in parliamentary practice the work of the Spanish Parliament (Cortes Generales) is conducted in various committees called comisiones parlamentarias, which are either comisiones permanentes (“standing committees”) or comisiones no permanentes (“ad hoc committees”). Each house of parliament has its own committees (comisiones del Congreso; comisiones del Senado) divided into “legislative committees” (comisiones legislativas) and non-legislative committees (comisiones no legislativas). There are also “joint committees” of the House of Deputies and Senate (comisiones mixtas del Congreso y del Senado).
In other respects, comisión de servicios is an expression in which comisión cannot be translated as either “commission” or “committee.” As used in Spain, comisión de servicios refers to the temporary transfer of a civil servant (funcionario) from his normal duties to serve in another sector of the civil service (función pública). Thus, estar en comisión de servicios means to be “on assignment” or “on loan” from another civil service department. This is also expressed (especially in BrE) as “being on secondment:” Está en comisión de servicios en el negociado de documentación (“He is on secondment to the document department”).
For an additional instance in which comisión and “commission” are not cognates, see this previous entry on contrato de comisión.