How to modify PrestaShop translations
In PrestaShop you can modify the translations of any installed language pack.
Log in to the backend of your store, click on the Tools tab and then on the Translations sub-tab. From the drop-down menu under Modify Translations select the section of the store and then click on the flag corresponding to the language for which you want to modify translations.
On the new page there are many fieldsets. To see the expressions in a particular fieldset click on its name. On the left side are the expressions in English and on the right side are the expressions translated in the particular language. To modify the translation of an expression, simply type the translated expression in the text field on the right. After you're done, click on the Update translations button.
Upgrading a language pack will overwrite any modifications that you have made to the translations of that language. Any modifications to the five default languages that come prepackaged with PrestaShop will also be overwritten during an update of your PrestaShop application. In this case, you have to export the particular language pack before the update and import it back after the update is finished.
For some more details and screenshots you can also check out the PrestaShop Translations Tutorial.