fixes and updates on es.json

Hi! Made some fixes and updates on the file (see below for details). My name is Martin and I am from Argentina. My native language is spanish. My country is under high inflation and find Xumm an useful tool to fight against it, keeping my life savings safe. I think that helping translating the amazing work you have done creating Xumm will spread the app to more people here in Argentina and in LatAm. Hope you find my contribution helpful! Best regards, Martin.

- Saw that the majority of sentences were written in 3rd person, so I changed some sentences that were in 2nd person to 3rd person.
- Fixed some bad/literal translations. e.g.: line 7 'back' was translated as 'espalda' (that in spanish means a person's back).
- Improved meaning and context of some sentences.
- Line 34: 'exchanges'. Noticed that is used in IOU's screen (Add Asset) to describe the issuers of the Assets. I think it's more familiar for spanish speakers to refer to them as 'emisores' (=issuers) rather than Exchanges. In this particular case, it happen to be both: issuers and exchanges (except Towo Labs). So, my proposal is to call them 'Emisores' in the spanish translation because it's more suitable to describe what those 'entities/parties' do with the assets listed.