The Spanish translation is atrocious (the older games too, but they had less text, so it was bearable), I've been searching old threads and apparently the only way to change the game language is changing the whole Windows language. It's still the same or are other, less annoying methods to change the game's language?

The Steam version of HGH lets you select your language in the general properties, the GOG Galaxy equivalent would be to Manage Installation->Configure->Language.