I think the issue readers, including myself, can have with that is if they need to run to Google Translame to get through a bit of dialogue. You can pretty much assume a term of endearment is something sweet and loving, and move past without needing to go see exactly what "watashi no kokoro" means. But if you're writing primarily in English, for readers of English, then you really should try and stick with one language except where it's a concept that can't readily be translated. If you're writing for Japanese speakers, you should be writing ござる, which is the polite form of "to be." I'm pretty sure "de" is either an example of inaccurate Japanese from anime use, or one of those hangovers from katakana, like daylight = deraitu.