EspaÃ±ol - es
The Castillian (European) Spanish is missing as language. For instance the term "Carro de pedido" and other terms sound foreign in Spain.
I don't know how many users you actually have in Spain and I am surprised that nobody ever posted this feature. May be they faithfully spend many work hours updating their own language file version, every time new functions are added? Still that does not make WHMCS culturally sensitive toward Spain, because these customisations do not make Spanish-es available as choice while Portuguese BT, Portuguese PT and Catalan are. Ouch, that hurts!
From a marketing perspective we really miss a goal there.