When I was in a situation somewhat like yours, I chose to mostly focus on learning and speaking German, and only read enough Spanish to keep it from fading from my brain completely. Speaking is a totally different skill than reading, but you can use reading to keep from completely losing your progress in Spanish.
I’ve learned or am learning four languages, and when I want to focus on just one, or I’m really busy and don’t have time to focus on any of them, I find that doing a little bit in each language every day signals my brain that all four are important and worth remembering. I might lose some ground—forget less-common words and get worse at speaking—but it’s quickly recoverable when I’m able to refocus later.
Specifically, the first summer I spent in Germany and Austria, I had all kinds of Spanish-German confusions, just like your example, “nein, soy inglés.” (Sí, ja, nein, no, y and und were the worst!) When attempting to speak German, my brain would offer up a Spanish word anytime I didn’t know or couldn’t remember the German word. As I learned more German and practiced speaking, that problem faded.
While in Europe, I continued to study Spanish a bit. In addition to using Memrise, I used the website Comics.com, which has several comic strips in Spanish. I subscribed to a bunch of them, so I was reading in Spanish every day even though I wasn’t speaking. By the time I left Europe, my ability to speak Spanish had gone completely out the window, but it came back within only a few days of returning to Central America. I was amused that whenever I couldn’t think of a Spanish word, my brain offered up a German one! (That tendency faded after a couple weeks of speaking only Spanish…)
Another thing you can do is use Clozemaster to “learn” German from Spanish. That’s a fabulous way to work on both at once. I love Clozemaster. It’s a rather new website (only a year old). It’s fun and easy to use, it’s simple, but effective, and I’m frequently surprised to realize how much progress I’m making compared to the progress I made (wasn’t making?) without it. You can also change your Duolingo settings to learn German from Spanish.
In brief, I recommend you read and study just enough Spanish that you don’t forget it completely. Focus on speaking and learning German. Use at least one resource that teaches German from Spanish.
