Each call to the API must have an authorization token, by which the API service will determine if the request is authorized. There are two methods to retrieve an authentication token:
Tokens are valid for 24 hours.
Our recommendation for machine to machine workflows where individual users are not involved is to use authentication via API Credentials.
For both authentication flows, the best implementation would be to generate a token once and store it, then only request a new one when the token expires. This way you will get the best performance possible, as your application will not make an extra request for every translation call (this extra request will add up after a while if the application has high translation needs).