to get the best jobs, very much like teaching English, which is something else you know nothing about.
You're right - I am not an English teacher/tutor/translator. My experience and knowledge come from being a Polish speaker, Polish citizen and having 3 family members who are professors/teachers along with the experience of many posters here who are teachers. For example, Doug advertises tutoring for 70 zl an hour. In order to make 15k zl a month, he would need to have 214 hour long tutoring sessions at 70 z's an hour within a month. Considering a work month is usually 160 hours (actually 173.33 if we're going by 2080 hours/12 for average month) and tutoring is a part time job, such a wage would be impossible.
But yes, there are people who are employed by corporations and small biz and even government to tutor, translate, etc. Those were the 'business english' people I referred to earlier which I also stated command higher wages than the average English teacher in high school/grade school and an English tutor of children and adults. However, they are not making 15k - such a salary is the top % of Polish salaries and is usually limited to those working in management, IT, high level government, successful small biz, finance, etc.
Those are the rates a good teacher can ask and easily get, as has been confirmed on this thread by people who have actually done the job (i.e. not you).
Again, there are people on this forum who are teachers, translators, etc. I pointed out Doug as an example as I am not an English teacher. Furthermore, labor statistics and surveys confirm that the average teacher in a Polish grade school/high school makes $500-$1k a month.
Even if you go on indeed.com and look for translator polish/English jobs in uk the most they pay is 20k pounds - which is more than they'd make in Poland.
To those people who live in Poland, ask a few people today how much Polish teachers, tutors, etc make. They'll tell you even as low as 2,200 zl for teachers, up to 4k if they're lucky for grade schools/high schools, $6-$20 tops for part time independent tutors, and around 5k 6k up to 8k with a lot of experience for translators.