Hi @bobi
Unlike some other visas, there are not specific language requirements for the working visa 18b. However, the embassy and BFAA have to **** at how plausible a person’s application is. If a person does not speak German but speaks English and has a job offer with an international company where English is the working language, then that person can receive a visa. However, if a person does not speak German and applies with a job offer to work as a German teacher, they will not receive a visa.
For the job of ZFA I could not find any specific requirements as regards language. For some professions it is clearly regulated. Foreign doctors, for example, must have knowledge of general German at B2 level and additional knowledge of medical German at C1 level.
Here you can see a recommendation of the dentistry association in one of the German states that a person starting an Ausbildung as ZFA should have B2 level of German:
https://***-zahnaerzte--sh-de.translate.goog/sprachkompetenz-ausbildung/?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp
It is therefore a requirement that you can show knowledge of German in order for your visa application to be plausible. The embassy staff can assess an applicant themselves up to level A2 if a person has a clear ability of German at that level and there is no German certification available.
While it is good that your employer has written something, it does not replace an official certification of German ability. In such cases where a test is not immediately available, the guidelines for the embassy are to provide you with a generous amount of time to take the exam and not to reject the application. These guidelines are found in what is called the Visa Handbook of the German foreign office. It is very long document but the pages 541-555 are relevant to proof of German language ability:
https://***.auswaertiges-amt.de/blob/207816/3cbc239c32b04e2af0ee2e28e9a9ec6b/visumhandbuch-data.pdf
The only further step I can think of is to offer to come and speak with the embassy officials in person at the embassy in order to demonstrate your German ability. I would, however, only recommend doing this if you feel comfortable that you have at least an A2 level of German and are able to show this.
Best,
Éanna