I'm not sure what Vietnamese is like, but I know that Chinese is very hard, and Thai looks very complicated.

Finally,my son had the flu.

Finally,my son had caught the flu.
That's why he have has to be absent from nursery school while for five days.
I was disappointed because he was going to join a recital of nursery school this weekend.
I was indeed looking forward to seeing it.
I can't help it.
Fortunatly,somehow he is has no fever.
I was wondering if he really has the flu.
For your information he have has not been vaccinated.

But my second son doesn't hasn't get got the flu yet.
I hope the flu of elder son doesn't infect the second son.

Keep practicing! Your vocabulary is okay, your grammar just needs a little practice!
Your word order is really good though!  Good luck. 

bad weather :(

It's the first week of this semester, but the weather is so bad.I say, it's not so cold but it's very glum and moist. So it's makes you feel very uncomfortable.And what's worse, it's will be raining these next four days according to the weather forecast.My god,I am going to be wet.
However,I think that it's will be a nice day after this bad weather ,ha ha


I think your English is good! You made a few mistakes, but those can easily be corrected, but what matters most is that I was able to understand you perfectly fine! :)