First of all, for the title of your post, I recommend changing it to: "Which language is the most difficult to learn?" or "Which language do you think is hardest to learn?" You should not ask both "how" and "which" in the same question; you should use only one of those two.
Second, a lot of people often say the "hardest language" is Chinese / Japanese / Korean / Arabic, but that is a very Western-centered viewpoint
In reality, the "hardest language" honestly depends on what the native language of that person is, and also which other languages that person knows.
For a Chinese speaker, Romance languages and German languages are hard.
For a Persian speaker, Arabic will be relatively easier to learn, because the two languages share a lot of common vocabulary and similar writing systems.
I speak English, Chinese, and Spanish, so Eastern Asian writing systems (Japanese, Korean) are easy for me, and Western languages in the Romance and German families (Portuguese, Italian, German, Dutch, etc) are easy for me as well. Languages like Arabic and Russian would be hard for me.