[po] Fix typos and issues with last Spanish translation update

1) pólitica -> política (misplaced accent)
2) fuente -> tipografia (wrong translation)
3) missing starting exclamation mark ('¡')
4) grave -> fatal ('grave' is translation for 'severe', not 'fatal')
5) retornar -> devolver (not 'retornar' which sounds more artificial)
6) Use of 'usted' when translating 'you' (more formal)
7) Updating some fuzzy translations (unmarking them as fuzzy)
Merge pull request #637 from knocte/master

[po] Fix typos and issues with last Spanish translation update