Partial translation and performance

I plan to use nopCommerce with English and at least one other language. I will not translate the Admin areas of the other languages.

My understanding is that if certain strings are not translated (i.e. missing for that language), the default (English) language will be shown instead. Is this correct?

If I do not translate the Admin area of the application, should I "copy over" the English translations to the other language(s)? Or will the application perform faster if I let the other language(s) "default" to English?

Never mind this question. I see now that all strings should be defined for a language, otherwise the string resource name will appear instead...