I want to adapt Japanese comics into novels in my own language and style online, without profit, and mark the original author and source. Is this illegal?

Although it looks "innocent", there are two problems that must be clarified.

First, adaptation must obtain the consent of the original author and relevant copyright owners. Although you don't directly benefit from the adaptation, people have the right to ask you to pay royalties. Moreover, this kind of acquisition not only refers to money, but also includes reputation, spiritual identity and so on. Only in practice, if the "damage" is not great and most authors are not serious, you can stop the infringement. That is, in many cases, reprinting and adaptation will be accompanied by a statement, which generally means clarifying the ownership of copyright, indicating the attitude of unintentional infringement, and attaching contact information.

Second, the network belongs to the public communication channel, and it is definitely illegal to spread original content without restriction. Although domestic websites often see a "... please delete it within 24 hours", in fact, if others can prove it, it will not affect the original author to sue you for infringement.

So, do it (assuming you do it well, you will be popular. If you don't do well, nobody will care about you. First of all, you should contact the source of the information (that is, the website or publisher where you read this cartoon). Second, you should attach a copyright notice and an exemption clause.