Are the last five lines of the poem "I'm explaining a few things" by Neruda more effective in English or Spanish?

This is a great question, which has as much to do with pragmatics and semantics as with the formation of the words and the effectiveness with which they command the attention of the reader. 

The study of "pragmatics" refers to the appropriate use of language in different contexts. In this case, the final sentences of the poem (in English) are: 

Come and see the blood in the streets.
Come and see
The blood in the streets.
Come and see the blood
In the streets! 

This is a call to action from the author to the readers, inviting them to see why he refuses to write about allegorical things, flowers, love, and other beauties. There is a bloody civil war going on; one which even killed Neruda's best friend, Federico Garcia Lorca. People are being killed everywhere, and there is no reason to appeal to the senses. Life is too cruel as it is. 

Form: Enjambment or "encabalgamiento"

Take a look at the Spanish version of the poem, and then look at the English version, again. Notice how the verses have been arranged in a specific way so that the reader can focus on what Neruda wants them to visualize. I darkened the first phrase, and italicized the second phrase, so that you can see how he breaks them down in the stanza. It happens the same in both languages. 

Venid a ver la sangre por las calles,
venid a ver
la sangre por las calles,
venid a ver la sangre
por las calles!

The poet uses this repetition in verse and in a variety of structures with double enjambment in a way that it matches what would be the rhythm of the noises heard in the streets when people scream, find a body, call for help, and hear emergency sirens blasting everywhere.

This enjambment also switches the message across. It says many messages in just one stanza that repeats verses. It mainly says the following sentences:

"Hey! Go look at the blood in the streets!" "Go look!"  "The blood!" "The streets!", "Go look at the blood!"

Fortunately, this was not lost in translation and it makes the poem work equally in both languages, creating the same sense of confusion, tension, and sadness. 

Perhaps this is the tragedy of it all: War is universal, and death is universal. The language of war and death are sadly understood, no matter how they are spoken. One small phrase, such as "go and look at the streets", carries a lot of weight when it is referring to conflict and battle. This is a way to demonstrate the incredible way in which words reflect the psyche of the times when they are spoken.

Therefore, the poem is equally effective in both English and Spanish. Neruda is so honest, transparent and powerful in his use of language that he could write anything, and the meaning will transfer from one language system to another, thanks to the richness of the theme.