I like how trees and grass become green, how flowers blossom and how birds sing their songs all around.

The repetition of "how" isn't strictly necessary, but without it, the parallel structure is slightly confusing as I would have expected it to be "how… become(s)…"

It's lovely to take a walk, if it's not too hot, and if I'm at home, I feel how a warm wind blows(ing) into my room.

Sometimes it's too hot in here, since we have a continental climate, that jumps from -40°C in winter to +40°C in summer.

People like to go swimming in lakes when it becomes hot in the cities.

I would prefer "the" to invoke the contrast between the cities as a whole and rural areas.

I think it's better to take a cool shower, than to spend your time travelling 50-100 kilometres, just to swim 20-30 minutes or maybe evenat most an hour.

"Maybe even an hour" is technically correct but does not fit the rest of a meaning, since it suggests something positive.

You can't bring the coolness of the lake water home with yourself at home.

See my earlier comment about "cities."

In the city area, we have a lake called Kopa, but it's quite dirty and not everyone would go there to swim.

I think, if the city government doid something about it, and cleaned it, and bringought in laws to restricting its pollution, it would be a great place for people to have a swim to cool off and havtake a brestak with their families and friends.

Since you used the conditional "would," the "if" clause needs to use the imperfect.

"Restricting" is probably also fine, but "to restrict" sounds better to me.

I like how trees and grass become green, flowers blossom, and birds sing their songs all around.

It's lovely to take a walk, if it's not too hot, and if I'm at home, I feel how a warm wind blows into my room.

Sometimes it's too hot in here, since we have a continental climate, that jumps from -40C in winter to +40C in summer.

People like to go swimming in lakes when it becomes hot in the cities.

I think it's better to take a cool shower, than to spend your time travelling 50-100 kilometres, just to swim 20-30 minutes or maybe evenjust an hour.

You can't bring the coolness of lake water home with yourself at home.

In the city area, we have a lake called Kopa, but it's quite dirty and not everyone would go there to swim.

I think, if the city government doid something about it, cleaned it, and bringought in laws restricting its pollution of the lake, it would be a great place for people to have a swim to cool off, and to have a rest with their families and friends.