Is my English strange?

I prefer summer over winter.
冬より夏の方が好きかなぁ。
Coz, in Japan it's already dark at 5 in the winter.
だって、冬の間は日本は5時には、すでに暗くなるから。
It is still not dark at about 7:30 in the summer.
夏はだいたい7:30分頃まで暗くならないよ。
But, my favorite season is autumn.
でも秋が一番すきかなぁ。

I made these corrections based on a more casual tone/register, in American English. Americans also use the word "fall" instead of "autumn" most of the time.

Is my English strange?

I prefer summer over winter.
冬より夏の方が好きかなぁ。
This is because in Japan it's already dark by 5 oclock/pm during the winter.
だって、冬の間は日本は5時には、すでに暗くなるから。
In the summer however, it is not even dark by 7:30.
夏はだいたい7:30分頃まで暗くならないよ。
However, my favorite season overall/my overall favourite season is autumn.
でも秋が一番すきかなぁ。

_______________________

Coz = Because

It's a good idea not to get into the habit of using slang at this point, it's best to stick to the full words whilst you're learning.

Good job, keep it up!