What is the best programming language to use for auto sms?

If I understood your question correctly, you want to automate your sms service periodically or on a schedule basis via API.

In that case you can use any programming language of you choice.

If you use java, you can use java scheduler to automate the sms.

Something like this: https://dzone.com/articles/schedulers-in-java-and-spring

Other languages also have same kind of facilities and it actually depends on the language you are comfortable with