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The module's text strings are written in English, but you might want French, Spanish or Polish shop owners to use your module too. You therefore have to translate those strings into those languages, both the front - office and the back -offices office strings. Ideally, you should translate your module in all the languages that are installed on your shop. This could be a tedious task, but Smarty and PrestaShop's own translation tool make it far easier.
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Now that we have a French translation, we can click on the French flag in the front - office, and get the expected result: the module's strings are now in French.
They are also translated in French when the back - office is in French.