Obviously teaching of other language will be more difficult.

There\'s really no difference. Learning a language is learning a language. The only thing that might be different is the age of the learner - your mother tongue is learned at a very young age, but you can learn a new language anytime.

The biggest difference between teaching mother tongue and teaching of a second language in most cases is the age of the person learning and the number of hours learning. When learning your mother tongue you start the first day you are born and you learn for all hours that you are awake. Teaching a second language, you have the added benefit of having a reference point (your mother tongue). You can learn for example that \"bonjour\" means \"hello\" which can be very helpful to learn things quickly. When you learn your mother tongue you learn through association only but in most cases you spend more time learning and have more hours of repetition which leads to inevitable fluency.