f the teacher has an atrocious accent, as is usual in my experience, stilted speech and antiquated to non-existent idomatic control of the language, this will be merrily passed down from generation to generation!
that's very true. I might have mentioned it before at some other occasion already. There are people who come here, they think they speak English because they learned it at school (not only in Poland, in other countries, too), but really they can't. They were taught by someone who learned their English decades ago from someone else whose knowledge was even more outdated, and they use funny pronunciation and outdated idioms. It's really
not helpful when they try to find a job.
First, it depends on the purpose of the tutelage: if the intention is to be conversational while visiting another country as a tourist and being able to order a meal at a restaurant or navigate through a city, non-native speakers will be sufficient instructors. If the goal is to be able to function as a member of a foreign society, permanently or temporarily, native speakers are necessary. For most people the best solution is access to both.
Here in the States at least, in order to be a board certified foreign language instructor at either the grade school as well as university level, one MUST be (and hiring is usally based upon such) from the country of the language being taught.
I really have no clue what the requirements are, we had German classes in college, the teacher was definitely not German, but maybe that's why I didn't learn all that much. Thankfully, this is such a diverse country, you can find native speakers of virtually any language here.