I just tested this and didn't encounter that issue. One thing to consider is that if you are an admin whose language is set to English, you will see English even when you log in as a client whose language is set to Spanish. The client will see Spanish when they explicitly log in to their account (i.e. when you are not an admin masquerading as a client).
Another thing to consider is that when the language has not been completely translated in Blesta, there may be definitions that do not exist in Spanish, and so they will fallback to English.