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.