Hello Marco! Thank you so much for asking a question. Unfortunately I have never worked for Japanese companies as Web developer as I'm based in Europe. I cannot answer the working environment in Tech companies in Japan, I'm sorry.
One thing I can say is that you need to know decentJapanese if you want to work for a Japanese company in Japan even though you are Web developer. I want this situation to be improved. Even though the person is a skilled worker, if that person does not know Japanese, the normal company will hesitate to hire that person over other Japanese people. I met several talented people from other countries who does not want to work for Japanese companies due to this situation.
I hope other Japanese developers can answer your question with more insight!