Maybe I didn't explain well. In fact, when I let the language by defaut, I have "Add to cart". It's ok. the problem is that the website will be for French so when the language change to Frence, I get "Ajouter au" instead of "Ajouter au panier". So I deduce that it's because of the length of the button then I think if I can modify "Ajouter au panier" by "Acheter", the text will appear whole.
And also, I want to translate also the others English Title of the theme such as "Best sellers", "New arrival". Do i have make the translation via poedit or wpml ?