add french translation

[JeanDavidDaviet]

This PR ad basic translation abilities. I've never tried nextjs before, but it seemed using next-translate package. It may be rough around the edges, but at least it gives a baseline to add translation.

The overall structure doesn't change. But the locale URL is prefix to the route. So for french instead of https://invoicedragon.com/templates, you can use https://invoicedragon.com/fr/templates

translating the pdf doesn't work yet though

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Name	Status	Preview	Comments	Updated (UTC)
invoice-receipt-generator	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 26, 2023 7:19am

[JeanDavidDaviet]

This PR helps with the issue #3

[LaniJ]

Thanks for this! I'll be reviewing it soon 😊

[kenzivoy]

looks great!