Lilly Palmer spoke English and was dubbed (in English, Spanish, Italian etc), in fact the entire cast were dubbed regardless of what language they spoke. The reason is because the film was being pitched for international distribution. Many films (including Blood and Roses, Baron Blood, Black Sunday etc) were shot the same way, using imported 'stars' (like Lilly Palmer) and utilizing local production crew. No actual 'official' language version of 'La Residencia' would exist unless you count it as a 'Spanish' film. Incidentally, the English language version, 'The Finishing School' contains footage cut from the Spanish version (obviously any suggestion of incest and the like met with disapproval from the Spanish censors at the time). If you are going to seek a copy I'd suggest trying to track down this version- even the credits are in English and the music track doesn't skip from print damage ala the DVD of the Spanish version.

She spoke English and was dubbed into Spanish.