The very first time he meets Emilia, she stumbles over her words, speaking in what would likely be overly-formal Japanese if her words actually matched up. She's confused and it's hard to make out exactly what she's trying to say, but it's no surprise. They're in America, and the poor girl has the short blonde hair and green eyes and Western features that let him know that Japanese isn't her first language.
What confuses him is why she's trying to speak it in the first place, when, for as long as he's been here, he's been mostly speaking English with his colleagues. Shingen knows the language well, and the fact that this new girl thinks she has to speak to him in Japanese, maybe because she's concerned or because she thinks she's being respectful, or maybe just because she doesn't know any better, is both flattering and adorable. Perhaps a little amusing too, but he refrains from laughing.
"It's a pleasure to meet you, Miss Emilia," he replies in clear English, and her eyes widen in surprise, the words probably sounding almost foreign to her simply because she wasn't expecting that response.
"Oh! Um...I'm sorry, did I mess my sentence up that bad?" she asks, looking worried, and that is particularly adorable. He's heard a lot about her, including her age, which places her a year and a half younger than his son, and even from that he would be able to tell that she's incredibly intelligent, to have achieved the things she has. But he's already been told in detail just how intelligent she is, and he's been told to "go easy on her", since she's so young.
He has his own intentions with her, however; he doesn't plan to give her a particularly hard time, no harder than he would give anyone else, but he isn't going to coddle her because of her age. If she needed to be coddled, he doesn't thinks she would have made it this far, and he doesn't think she'll appreciate being treated like anything less than a coworker of nearly equal standing.
That said, he can tell from the way she asked her question, and from the nervous, apologetic look on her face that she knows her Japanese is terrible and that she's a bit self-conscious about that. Her eyes look worried, like she's afraid she may have offended him in some way, and he can tell that this isn't the first time she's struggled in a situation like this. The fact that, despite all that, she tried for his sake, makes the situation incredibly flattering- and perhaps a bit more adorable as well- and he can't help himself. He has to go easy on her, at least a little bit, at least this once.
"No, not at all," he replies. "You spoke my language, so I'm returning the favor and speaking yours. That's all!" Her smile is very much worth the lie.