This is a really good way to practice, I did this when I first learned Spanish in middle school.

Okay for the corrections, I put them in parentheses and my comments are in italics. 

(A) few days ago, Spain had (an) unfortunate accident.  <--Something I have noticed is that many Koreans don't use indefinite articles. Korean does not have them, I know, but in English, we never send nouns to a party naked. That means, there is always something before the noun. 

According to the report, the train was travelling well in excess of (over) the 80 kilometers an hour speed limit when it crashed. <--We don't typically use the word excess to mean "more than" in this particular context. Also, if you mean that the speed limit was already 80MPH and he was driving faster than that, you might want to simply say, "the train was going well over the speed limit." The actual limit itself isn't important.

When I heard about (the) accident in Spain, I feel (felt) sorry. That's because, my favorite country is Spain. <--We also don't use "sorry" in this way either. It would sound more natural to say, "When I heard about the accident in Spain, I was saddened," or something like that because "sorry" gives it a guilty or pitiful feeling that I'm sure you don't want. 

Anyway, I read an article (in which) about what (the) Prime Minister of Spain said,
"Today I will sign a decree declaring three days of official mourning in all of Spain." He said. <--We can join those phrases together by using "in which," so that way you don't have to repeat "he said" at the end.

This declaration is impressed impressive. <--A declaration cannot be impressed, that is a past participle that functions as a adjective for people, impressive is the adjective for nouns, including gerunds which also act as nouns sometimes. 

(The) government and all (of the) people in Spain mourn for (the) victims.

What a impressive behavior ever! <---The indefinite article is not needed because you are generalizing. When we generalize, we don't use articles. Like, "Cats are evil," I did not say the cat or A cat because I am generalizing all cats. Also, see how you used impressive instead of impressed! 

In (the) case of my country, I make sure our behavior.<---I would just say, "In my country..." I'm not sure what you mean by that last bit though.

When the accident had happened, we might be want to know the cause first. We just want to know 'Why accident happened'. Not a mourn for victims. <----"If an accident were to happen in my county, we might want to know the cause first. Instead of mourn the victims, we would want to know "Why did this happen?"

I changed when to if because "When" is in the indicative mood, that means that you are certain this accindent will happen. I put "if" because this statement sounds hypothetical, or subjunctive. Meaning, it has not really happened in real life. Also, I put those phrases together and added "instead" because separating them and saying "No" sounds a little irreverant. 

So, I think we should follow (the) behavior of (the) Spainish. How to express our condolences to victims.

This was really good. I envy you. I can't read Korean papers at all. I hope my corrections made sense!