Did you downloaded the proper language pack?
For 7.8 you can use a language pack for 7.9 but not from the 7.10 version
SuiteCRM versions: 7.8.x and 7.9.x = Download ZIP from Sourceforge:
If errors are not solved, some questions for you:
What is your 7.8 subversion?
Are you able to make an “Admin / Repair / Quick repair and rebuild” without errors?
Are you installing the proper way (Admin / module loader/upload the spanish zip)
Is your system working fine (permissions levels)? You can test this by going to Admin / Upgradewizard / and perform the test step (no need to make an update for your system, just the testing step)
Note: The string involved says:
'ERR_UW_CONFIG' => 'Please make your config.php file writable and reload this page.',