Corrections in red.  Stylistic suggestions in blue.

Election Day

Today is the Colombia's presidential election day. Almost 20 days ago the "first round" took place. Since none of the five candidates got more than 50% of the votes, a second round has to be held between the two candidates who received the most votes. A problem occurs when neither of the two, in my opinion, are good. The only choice is not to vote for the best candidate, but to vote for the "lesser of (the) two evils".  [Alternative:  ".... but to vote for the one who is less bad."]
 
P.S. I know I had been trying to learn Dutch, but I think I will improve my English first, because the most of the resources for learning Dutch are written in English. I hope I'm doing the right thing.

Election Day

Today is the Colombia's presidential election day. Almost 20 days ago was The "first round" was almost 20 days ago. As none of the five candidates got more than 50% of the votes, the election must be repeated for  between the two most voted candidates with the most votes. The problem is that, in my opinion, neither are suitable. when none of both are, in my opinion, good. The choice is not to vote for the best, but to vote for the " least worst less".

P.S. I know I had been trying to learn Dutch, but I think I will improve, firstly, my English first  because the most of the resources to learn Dutch are written in English. I hope I am doing the right thing.to be doing the right.

Election Day

Today is the Colombia's presidential election day. Almost 20 days ago was the "first round"/It was almost twenty years ago that our country had the "first round". As none of the five candidates got more than 50% of the votes, the election must be repeated between the two most voted for candidates. The problem is when none of both/either are, in my opinion, good. The choice is not to vote for the best, but vote for the " worst less".

PD PS. I know I had have been trying to learn Dutch, but I think I will, firstly, improve, firstly, my English because the most of the resources to learn dutch Dutch are written on in English. I hope to be doing the right thing.

Excellent, Juan! I also like the parentheses you've put in there! Things in red are incorrect, things in green are what I think they should be or other little things I've added in and things in yellow are things that I will talk about here

-Countries never need an article (Apart from when talking about "the UK" or "the USA"!)

-The first phrase in yellow I think makes sense, but it sounds clumsy. It's best to use what I've written in green

-"the most voted for candidates" - because you "vote for" someone in an election or "the most popular candidates"

-The phrase "none of both" doesn't sound right. You can use the word "neither" which basically means "none of both" or "none of either" - although that still sounds odd. It's best to use neither

-When you write an extra little bit on the end of a letter or post then you write "PS" which stands for "post script"

-You've put the parentheses in the wrong place here, it often comes after the finite formation - which is the subject + the finite verb. In this case, your subject and finite verb are "I will"

-All languages are proper nouns in English and therefore need a capital letter

-Things are written in English.

-I don't really understand what you mean by the last sentence, and have added in the word "thing" because it makes more sense, although I've had to guess at what you meant.

Hope this helps! Keep studying hard! :)