Today is 20th of July,July 20th, Colombia´s Independence Day of Colombia

typical structure for dates/holidays

So, they planned to start a public fight, and then justify the creation of a local government.

That comma was grammatically unnecessary, but it would be okay to keep it there for stylistic reasons.

The vocal pause is unnecessary when reading that part.

Their goal wereas to make citizens fight against abuses and xenophobia, but they needed an excuse.

"were" would correspond with two goals, but there is basically one here: to prompt citizens to fight (against two things: abuses and xenophobia)

Then, they sent Luis de Rubio to Llorente's shop.

No grammatical issues, but it is customary to use the first and last name when introducing a historical figure for the first time. After that, then only the first or last name is used. For example, since this is the first time mentioning Llorente, you could say something like:

Then, they sent Luis de Rubio to the shop of a Spaniard named José González Llorente. (I included "Spaniard" to show how this can include the next sentence/idea)

Llorente was a Spaniard man.

grammatically correct, but unnecessary if you choose to include it in the previous sentence

Luis asked for a favor, he and requested a flower vase to decorate a table.

more common phrasing, although your initial sentence might be heard from certain speakers with a certain style

His excusereason was that an important man was arriving for lunch.

"excuse" has the connotation that it was already known that he was being dishonest or ill-intentioned, but at this point in the story, the reader doesn't know that the request was a ploy.

That correction is technically optional, but if the reader is unfamiliar with the events, "reason" gives a different, more neutral feeling.

Llorente didn't accept to do the favor, and also told Luis xenophobic comments about Criollos.

unnecessary comma because the "also..." clause is dependent

Could be okay for stylistic purposes.

You might also say, for a bit of a more natural feel:

Llorente refused to do the favor...

Llorente wouldn't do the favor...

Llorente did not oblige...

The scene was planned,; they knew that Spaniards hated Criollos, and they knew this favor was impossible.

the semicolon kind of acts more like a period than a comma here

He argued and made the situation worste.

"worst" is the highest level when comparing more than two things

Something can be bad.

Something more bad is "worse" than the first thing.

Whatever is the most bad, even more than the "worse" one, is the "worst."

bad--worse--worst

like good--better--best

I guess in Spanish "worse" is "peor" but "worst" is "el/la peor"

In the middle of the chaos, Los Criollos took advantage of the opportunity and gathered people to shapeform a Criolla Board of Governors.

"form" is used commonly

Also, most would say "the Criollos."

Nowadays, the Flower Vase is kept in the Independence Museum, in the capital, Bogotá.

another option that has a slightly different feel: "Nowadays, Llorente's Flower Vase..."

Today is the 20th of July, Independence Day of Colombia

Or, “Today is the 20th of July, Colombia’s Independence Day”

The independence of Colombia took place 215 years ago.

Or “Colombia’s Independence Day ….”

Everything started in Santa Fe de Bogotá, the capital of Colombia.

Then, they sent Luis de Rubio to Llorente's shop.

Llorente was a Spaniard man.

Or Llorente was a Spanish man.

Luis asked for a favor, h. He requested a flower vase to decorate a table.

Llorente didn't acceptgree to do the favorrequest, and also told Luismade xenophobic comments about the Criollos to Luis.

The scentage was planned, tset. They knew that the Spaniards hated the Criollos, and they knew thise favor was impossibleould not be granted.

I don’t understand, “The scene was planned.” Do you mean, “The stage was set.”

So, when they were in the middle of the fighting, Antonio Morales (the Criollo planner) appeared to increase the hatred.

Soon, the fighting attracted many people.

In the middle of the chaos, Los Criollos took advantage of the opportunity and gathered people to shapeform a Criolla Board of Governors.

The Flower Vase was thea sign of rebellion, and the 20th of July is remembered as Independence Day in Colombia.

Or, “The Flower Vase was a symbol of…”