You can enable creating and sending of invoices in the language your customer used while shopping in your store by adding a unique code snippet to the cart file in your theme.
To install the code snippet to your store:
Step 1: In your Shopify Admin, go to the Online Store → Themes page.
Step 2: Click the Actions button, and then click Edit code.
Step 3: Select cart.liquid in the left sidebar. You might use the upper search bar for a faster search.
Step 4: Insert the following code snippet before the </form> tag.
Step 5: Click on the 'Save' button to save the change.
<!-- PDF Invoice app -->
<input type="hidden" name="attributes[Invoice Language]" value="fr"/>