Incredibly important-- but it's hard to find historians who have the money to hire an outside translator. Most have had to do some language training as part of their PhD (mine required proficiency in two languages) which is usually good enough for working with archival sources.

Sometimes historians shift or switch fields later in their careers and may find that they lack the language proficiency they would like-- but again, if they can they tend to gravitate towards graduate students that can be employed at a bargain price in exchange for a line a CV or a Letter of Recommendation (guilty of accepting this kind of work myself). Occasionally they might reach out for a professional translator, but, it really depends on how much material they really need translated and whether or not translations already exist.

One fruitful avenue that does come to mind is translating articles/monographs between languages. Assuming your first language is English, then you could definitely inquire with publishers about translating Spanish monographs/articles into English-- or at least getting on their call sheet for when they need translators to translate abstracts and etc. I am not sure of the demand for Spanish, however, I do know a few French colleagues who stay pretty busy with translation projects involving history monographs.

PS. Congrats on the soon-to-be graduation!

I would expect command of Spanish to be near universal among historians who work with Spanish primary sources. I've also been told that the pay for commercial translation is far better than academic translation.