Carole Lombard had long been popular with Spanish-speaking audiences, as she was a magazine cover subject for two of their magazines in 1929. (The above cover is from October 1937.) Aiding her appeal was the Hollywood publicity machine, whose work went far beyond the English language.
We have proof through a Paramount in-house publication, Mensajero Paramount, of which some copies from 1937-38 have been placed online. In fact, here's Carole and several of her studio star cohorts on the cover of the January '37 issue:
Paramount released two Lombard films that year. The first, "Swing High, Swing Low," was teased in the January issue...
...then in May's:
"True Confession" received similar treatment in the January 1938 issue:
On both films, Paramount management provides some marketing tips for exhibitors...and this is where I wish I had taken six years of Spanish in school, not six years of French. (Those of you adept in Spanish are encouraged to translate some of this for our enlightenment.) However, I do notice references to "fotografias" with the number "1712," which happens to be coding for "True Confession." Here's 1712-14, with Lombard and co-stars Fred MacMurray and Una Merkel:
Here's some other samples of Mensajero Paramount -- the May 1937 cover, featuring Ernst Lubitsch and Marlene Dietrich, promoting their new film "Angel"...
...and an inside page from March '37 (note this covers both Latin America and Spain, as well as Portugal and Brazil, where Portuguese is the dominant tongue):
Unfortunately, the only other issues that have been made available are from 1927. Let's hope more volumes are found; one wonders if the publication has any information (or pictures) on Lombard's trip to Cuba in early 1935.