It's not clear from your post, have you tried Gumroads API? They have an endpoint for verifying licenses that works nicely.

  curl https://api.gumroad.com/v2/licenses/verify \  -d "product_id=SDGgCnivv6gTTHfVRfUBxQ==" \  -d "license_key=YOUR_CUSTOMERS_LICENSE_KEY" \  -X POST

Example in React/TS:

  const checkLicenseKeyWithGumroad = async (licenseKey: string) => {
  try {
    const isValidLicenseRequest = await axios.post(
      process.env.GUMROAD_VERIFY_URL as string,
      {
        product_id: 'SDGgCnivv6gTTHfVRfUBxQ==',
        license_key: licenseKey?.trim()
      }
    )
    const isValidLicense = isValidLicenseRequest?.data?.success ? true : false
    return isValidLicense
  } catch (error) {
    console.log(`Error checking license key: ${error}`)
    return false
  }

}

For your language issue, you could self-host the platform and only run purchases through Gumroad. This is how I do it.

https://docs.lemonsqueezy.com/help/licensing (I've never used it myself)

Long term HN user @ezekg also runs this https://keygen.sh/ if that might suit your needs (i.e. if you want to separate out licensing logic from the payment logic)

Highly recommend implementing accounts for your software and licensing through that, as well as making some features of your app online only (server side), even if they don't have to be.

If your software becomes commercially successful it will eventually get cracked and many corps will use that because licensing in an org setting is messy and who wants to ask for software or deal with the red tape?

>also support EUR as a currency.

Doesn't Gumroad support EU (I assume you mean Euro) as a currency?

EU & UK VAT on Gumroad

https://help.gumroad.com/article/10-dealing-with-vat

Fastspring or Paddle.

Steam?