I find grammatical mistakes in non-native speakers endearing. Either when they speak English and are non-native speakers of English (I am too), or when they speak my native language and they are not native speakers of mine.

Especially when it’s apparent that it comes from how you would phrase something in the original language of the person speaking/writing.

Or as one might say: Especially when it is visible that it comes of how one would say something on mother’s language to the person that speaks or writes.