Phrase Request: "electronics for military use"

An international military electronics company

This seems to be the most direct translation. I would avoid using the phrase "defense contractor", since that specifies a relationship between the company and a government (typically the United States government), which might not be the case here.

/An international company operating in the industry of electronics for military use/ should read: An international company in the military electronics industry. The word operating sounds like a construction used in a Romance language; it is not needed in English. Typically, in English, one says: an x company in the y industry. Military electronics is an industrial sector.

I think your second sentence needs to be rearranged:

an electronics contractor for the defence sector

Your version suggests that the company provides security services to the electronics industry.

You could go with:

a contractor for the defence electronics sector

Or, most succinctly:

a defence electronics contractor

Honestly I would stick with your first sentence. Also, ensure the correct spelling of defense/defence for your target if you choose to go that route. American English would use the s, while British English would use the c.