I am a player who speaks Spanish and when I start a game it is in English. I wanted to know if there is any option to change the language in Gog Galaxy?

Hypothetically, maybe.

The reality is, localization is hard, and getting anyone to take localization for places like SA seriously is something that even billion dollar franchises have ignored. (Pokemon.)

When I go to my 400+ collection and set the language to Spanish, there's maybe two pages of games.

As to the matter of the Galaxy Client? Pssh, I don't use it.

It depends upon the game. Some games allow changing language from within the game itself, while others require running a separate settings/configuration program that will usually be located in the same directory as the game executable. Then there are those annoying titles that require you to install the localization version you want, and if you change your mind, you have to reinstall, this time using the other localization you decided on. There are other possibilities as well--like those asshole games that have their language locked to whatever your operating system's language is set to.
Mention the game you were wondering about, and someone may be able to tell you how to do it (assuming that the GOG version has Spanish as an option, of course).

Does the game in question even support Spanish in the first place? Its storepage should show you what languages are supported.

If not, then you are out of luck.

If it does, then it depends on how the language options are implemented. Some games will allow you to change languages during installation. Other games will allow you to change it from the game's options. Some others will be of the annoying kind and automatically change the language to match your operating system's settings, giving you no option to change it to your preference.

1. Click the "More" button to the right of the Play button, then choose Manage Installation, then Configure.
2. In the window that opens, you can see if you can switch the language in GOG Galaxy or if that's handled in-game.