Any languages

Korean is similar to Japanese in many ways. It's also a fun language to speak. But I don't think many people outside Korea speak it.
Italian is also good. I like it more than Spanish.

個人的な興味によって違うかしら。自分は言語全体が面白いだと思っていますので、新しい言語を勉強すればするほど面白くなります。単語や文法、自分がならった言語に共通点があるかどうか、それを考えるのは好きです。たとえば、自分はドイツ母語者なので、同じゲルマン語派に含まれている言語（英語・スウェーデン語等）の共通点で覚えるのは簡単になります。この間は中国語を習い始めましたが、日本語を長年習ったおかげで中国語の漢字を覚えるのは簡単で、文語もちょっと習ったおかげで文法が分かりやすいです。

三つのやり方があると思います。一つ目は、日本語に近い言語（韓国語・中国語等）を挑戦すること。二つ目は英語に近い言語（ドイツ語・オランダ語等）を挑戦すること。三つ目は、まったく関係のない言語を挑戦すること。三つの中では一番ふさわしいのは自分で考えなければなりません。

I think it depends on what you're interested in. I find all languages equally interesting and the more I study a language the more interesting it becomes. I like thinking about whether a language I'm learning has similarities to any of the languages I learned before. I'm a native speaker of German, so for me learning languages in the same language family (like English or Swedish) is easier. I started learning Chinese recently as well, and since I've been learning Japanese for quite a while, learning the kanji is easy and as I also learned some bungo, even the grammar is easy to understand.

I think there are three ways of doing this: 1. You tackle a language that is close to Japanese (like Korean or Chinese) 2. You tackle a language that is close to English (like German or Dutch) 3. You tackle a language that has no relation to either. Which of these three ways is best for you you'll have to decide for yourself.

Korean is probably the closest language to Japanese there is.

French is probably the closest language to English.

I think to learn a language well. You need a lot of motivation and interest.
Is there any country that you visited and really liked it?
Any culture that fascinates you?
Any movie that you absolutely love?

If you like classical music, you might enjoy learning Italian.
Art, cuisine and fashion >> French
Philosophy, law >> German

Just some ideas :)

Yosa-san, I have spoken English and Spanish since I was a child, so I became interested in French when I was a teenager (it's a Romance language like Spanish). Later I became interested in Italian because it's similar to Spanish.

Then I became interested in German culture, so I learned some German. (English has many words of Germanic origin: Wasser (water), was (what), Vater(father), Mutter(mother), Sohn(son), Tochter(daughter), etc.

When I wanted to travel to Greece and the Balkans, I taught myself Greek and Bulgarian.
Since Bulgarian is similar to Russian, I also began to learn Russian.
When I travelled to Germany and Austria, I listened to Czech TV and becamse interested in that language.

As you can see, there are many reasons why I have learned languages :)

Spanish is a cool language, but it can be tricky. Just choose whatever you think is cool.

Well, you can try learning chinese cause many people speaks chinese. Plus, chinese is a little similar to japanese.