English language title and keywords in Spain contributor account

Language depends on the end user's choices.

To access Adobe Stock in Spanish, go to stock.adobe.com, scroll down to the bottom of the page, and select "Change Region" to choose your preferred region and language.

Categories are not used in the search. 

What you are seeing in Spanish is the interface (user experience) that is in Spanish. When you change your region, you will see those in your new region's language (let's say Italian for Italy, French for France). 

What you enter as keywords is translated into the regions language for the potential buyer. The title, however, stays in what ever language you initially used. So, I think that it is wise to use English for that. You just need to stay attentive for typos. I use a language tool to check my entries. You can check that, by changing your region for a moment into a different language.

Your assets are presented to all countries where Adobe stock is available, so your sales are worldwide. 

If you set keywords with Spanish settings, they will appear at the top of the search results when a search is conducted in Spanish.
This is because they are optimized by language.Conversely, if a search is conducted in English, assets with Spanish settings will be ranked lower in the search results.

Some assets may be popular only in certain countries or regions, so it may be a good idea to change the language settings depending on the asset.