Hi José

Translations are a bit of a problem in Odoo.
You could translate them through the settings (Settings > Application Terms > Translated terms) and they will be translated but there is one downside. When you would create a new database those translations would not be added since they are stored as items in your postgreSQL database. Only through a back-up of the current database or a duplication the translations would stay preserved. 
The second option you have is to make the translations in the .po file (for your specific language) from your module and update those but this seems to be a bit of a hit and miss.. They don't always seem to work.
A third option is that you contribute to the translations of your language on Launchpad (see https://translations.launchpad.net/openobject-addons/8.0/) in theory if everything is translated there so should it be in Odoo. Its not 100% but you'll see does its job rather good.

The best way is hard to say and is a bit of an user choise.. I usually just go with adding the translations through Settings > Application Terms > Translated terms since that is the fastest and always works. You could even generate CSV files for this, so you can always re-use them.