Could you explain why we should add the es-ar ? what are the differences between spanish and spanish argentina?

So this is the story: if one would be somebody that doesn't understand a bit about Spanish language stuff, I'd begin telling that buddy there's basically 2 different Spanish "branches", let's say The Spaniard and the Latin American one. Spanish (Spain) has a different treatment, as in speech as in texts. This different treatment is bigger in informal ways, as in formal modes differences are less. So, I'm assuming you ask people to localize Piwik in an informal/familiar way, not pure slang nor totally serious & polite, right? If that's correct, so, yes, differences between Europe's Spanish & Latin America's Spanish are big. That's not the end of story though.

Now let's zoom in Spanish speaker Latin America only. There's a few differences between Mexico's Spanish and all the way down till Bolivia. However when you are in Argentina, all the Spanish you have learned is less than useless. Spanish (Argentina) has a different way of conjugating verbs and subject pronouns change as well [t & vosotros are Spanish (Spain) for you second person, singular & plural pronouns respectively, while vos & ustedes are Spanish (Argentina) for you second person, singular & plural pronouns respectively. From that point on everything changes, as actions have a different spelling. For instance: You think is Tu piensas in Spain & most Latin American countries, but in Argentina it's Vos penss. This is just a silly example diffs are deeper: Spanish (Argentina) has a lot of slang within itself. And this is something inescapable, it's part of the Argentine Spanish and its own roots. You will want to have a look at Che, boludo! book if interested to know more about Argentina's Spanish.