Update to Spanish translation

I came across your app in F-Droid and I liked it, but I couldn't help noticing that the Spanish translation was quite lacking. So I updated it to get rid of the untranslated text and made the track descriptions more gramatically correct.

I'm not sure how to call the tracks though, as I don't know what you had in mind when you picked the name. For now I picked the Spanish 'pista' as in a race track or runway (it also happens to mean 'clue') because it's the most direct translation. But maybe you wanted something more like 'way' and 'journey', or maybe like 'footpath', or perhaps like 'record' and 'trace'. Let me know and I'll update it.

Cheers!