Is learning the Japanese language for work or study considered a sin, if most of those who learn it use it to watch the forbidden "anime," knowing that my intention is to learn for worldly benefit?

Learning and teaching foreign languages are permissible, and a Muslim may be rewarded for it if it results in a religious or worldly benefit. However, teaching them to someone who will use them for forbidden purposes is not permissible, due to the Almighty's saying: ﴿And cooperate in righteousness and piety, but do not cooperate in sin and aggression.﴾ So, whoever knows or strongly believes that the learner will use them for forbidden purposes, it is not permissible to teach them. As for one who does not know that, there is no harm in teaching them. The basic principle in learning is permissibility, and there is no obligation to ask the learner about the reason for their learning.