in questo caso un'insegnante di nazionalità giapponese

The English sentence is ambiguous. In English this ambiguity is due to the fact the adjective (Japanese) is put before the noun (teacher) with no indication of case (either by word ending or preposition). In Italian there is no such ambiguity:
Sono un'insegnante di Giapponese = I'm a Japanese language teacher = I'm a teacher of Japanese
because we use "di", meaning "of" here it means that you teach Japanese

Sono un'insegnante Giapponese = Sono un'insegnante del Giappone = I'm a teacher from Japan
here "Giapponese" is clearly an adjective denoting nationality and referred to "insegnante"

As a side note, insegnante is invariable:

a male teacher = un insegnante
a female teacher = una insegnante or un'insegnante

The apostrophe signals the gender in writing. In speaking if you elide the article, the gender remains ambiguous, if there is no context to tell you.

I think it would be better to split this sentence in two, one for each meaning.

I mean that the English sentence "I'm a Japanese teacher" is a bit ambiguous, does "Japanese" refer to the language taught or the teacher's nationality? It indeed translates into two different sentences in Italian. Here are all their variations:

(1) Japanese means nationality
"Sono un'insegnante Giapponese" (feminine)
"Sono una insegnante Giapponese" (feminine)
"Sono un insegnante Giapponese" (masculine)
"Sono un'insegnante del Giappone" (feminine)
"Sono una insegnante del Giappone" (feminine)
"Sono un insegnante del Giappone" (masculine)
"Ich bin Japanischlehrer"

(2) Japanese is the language taught
"Sono un'insegnante di Giapponese" (feminine)
"Sono una insegnante di Giapponese" (feminine)
"Sono un insegnante di Giapponese" (masculine)
"Ich bin ein japanischer Lehrer"

I suggest that meanings (1) and (2) should be kept distinct. I would group sentences meaning (1) in all languages in one page and those meaning (2) in another page. The English ambiguous sentence can be included as a translation for both (1) and (2).

This is an English problem, and maybe a problem of other (almost) case-less languages, not a German or Italian problem.

The japanese sentence says teacher of japanese language

Grazie mille! I understand. But 'Ich bin ein japanischer Lehrer' still has some direct (green-arrow) Italian translations which are wrong like "Sono un insegnante giapponese". It should be "Sono un insegnante di giapponese". That 'di' is needed because "Sono un insegnante giapponese" means "Ich bin Japanischlehrer". I guess I have to post this remark to the owner of the wrong Italian translation for a correction, not you. Is that correct?