Well, I don't think there's much to add to what you've already said. I have never been in your position, but if I had to sell the idea, I'd start with the same arguments:
Most third-party libraries will not come with a Spanish API, so you might as well have the whole codebase in English for consistency.
The future is hard to predict. In five years time you may find that you have a non-Spanish speaking employee that of course needs to understand the codebase, or maybe you need to off-source some development to another country, or even an international company might acquire your company and all of a sudden a bunch of Germans need to be able to read your code. In all cases Spanish isn't the ideal choice.
Even though English is not my native language, virtually all programming-related content I consume (books, videos, tutorials, blog posts, etc.) is in English, so programming in English rather than my native language is actually easier for me. I know there's plenty of content in Spanish, but I can imagine many native Spanish speakers are in a similar position.
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Well, I don't think there's much to add to what you've already said. I have never been in your position, but if I had to sell the idea, I'd start with the same arguments: