I believe you can to go set up your keyboard so that Spanish shows up as one of your options in the System Preferences. With the text box selected that you want to edit in OmniGraffle, make sure Format>Text>Allow Hyphenation is turned on, and the Spanish keyboard is selected,
Hopefully this will be enough to allow you to use Spanish with the main computer setup in English. You may also want to tweak your Language and Region settings in System Preferences to make sure as many defaults match your content as possible.
This seemed to hyphenate with Spanish phrases in my test here, but since I am a novice Spanish speaker, I’d love to hear back if that is setting the hyphenation as a native speaker would expect. If you’d like OmniGraffle to be in Spanish while the macOS is set to English, use the preference omnigraffle:///change-preference?AppleLanguages=(es).