Although I agree with some of what you are saying, you are very unreasonable.
I cannot understand how somebody who works with languages would seek to obliterate mother-tongue from another human being. You of all should know how important language is for personal, and cultural identity, especially if living abroad.
What is it really about he speaking Spanish that you dislike?
I am Norwegian, my husband is Polish. I have lived in the UK since I was 21, he since he was 18. We both speak fluent English, we speak English together and with our children (who are English Norwegian bilingual)
My husband also speaks Norwegian. We lived in Norway for 3 years, and he did a 1 year Norwegian course. His Norwegian is not as good as his English. Our relationship work better in English than in Norwegian, but this is mostly due to having dated, fallen in love, and gotten to know each-other "in English". Maybe it would have been different if we met in Norway and he spoke Norwegian? The second reason is that the limitations of his language, although fluent to chat, to go shopping, etc, falls short when discussing budgeting, finance, our children, and life issues. We instantly change to speak English together. It is just easier. I dont speak any Polish. He speaks English, Polish, Norwegian, and understands some Russian and Czech. I speak English and Norwegian and understand Swedish and Danish. We cant "wish away" any of the languages.
I have a friend whose mother is Belgian and her Father is Moroccan. Her father never spoke Arabic at home. My friend feels that she has lost some of her identity, culture and heritage, through her father never speaking to her in Arabic, because there is so much identity tied up in the language.
When you look at indigenous populations around the world, like for example the Sami people in Norway, who suffered language oppression, and were forced to speak Norwegian as opposed to their native tongues, this has caused all sorts of identify problems.
So, seeing there is no practical reasons why your dp should speak English all the time, I ask again, what is the real reason you want your dp to only speak English? What is behind this? I think you should examine this carefully, and also whether you and her have a future together, when you are so opposed to something so quintessentially her, her Spanish language and heritage.