When Eren wakes up in the morning he slips into his German tongue, he confused Squad Levi so much because when they got him out of his cell every morning they had no idea what he was saying, until he explained after he woke up that he always did that. Levi started being the one to wake him up after that.
Eren getting so angry that he curses someone out in all German and they`re terrified and offended at the same time because they don`t understand.
Him having to attend a capital party because Levi went and somehow saves an investing deal because he`s the only one who could translate for the German noble, Erwin almost hugged him, but Levi did it first.
Eren likes to whisper sweet German nothings to his captain in front of everyone, no one else understand but Levi knows the phrases he`s saying all too well, Levi is blushing and no one knows why. Isn`t Eren just talking about the weather?
When Eren speaks German his voice gets deeper and rougher with the language, making him sound older and to Levi, heavenly.
But the best is when Eren slips into German without realizing it so he`s casually speaking it without realizing no one understand him, then he notices he`s not even speaking their language and blushes.