PThe pitch always varies.

I really like the English language because of the way its sounds.

PThe pitch always varies.

Another correction that someone made is "It's pitch always varies." This actually has a typo; it should be "Its pitch always varies."

The Japanese language sounds so flat, doesn't it?

I think this sounds more natural.

The English lLanguage

I really like the English language because of how its sounds.

PIt's pitch always varies.

PThe pitch always varies.¶
or ¶
You could combine the sentence above and this one by writing: "The English language sounds so beautiful to me because the pitch always varies".

TIn contrast, the Japanese language sounds flat, doesn't it?

Adding "In contrast" or any other contrastive (=opposition when you compare two different things) conjunction such as "On the contrary" makes the text flow better when comparing two things (in this case you are comparing the English language to the Japanese language).

The English lLanguage

PThe pitch always varies.

PThe pitch always varies.

PThe pitch always varies.¶
or¶
The pitch is always varying.

The Japanese language sounds flat, doesn't it?¶
or¶
The Japanese language sounds flat, don't you think?¶

The second would sound a bit more natural.