This can be done in both automated and manual ways.
Looks like, you already know how to add the header to each request manually.
In order to be able to do it in automated fashion, it requires 'Events' feature which is available out-of-the-box in Pro editions[ReadyAPI/SoapUI Pro].
However, for open source edition, there is an extension available here, please follow the instructions.
https://github.com/nmrao/soapuiExtensions
Once you followed the instructions, need to add the code given below to in %SOAPUI_HOME%/scripts/RequestStepBeforeSubmit.groovy file to add the custom http header. So that above extension would execute this groovy file before submitting the request.
Here is code that goes into above groovy file:
if (submit.request.testStep instanceof com.eviware.soapui.impl.wsdl.teststeps.RestTestRequestStep) {
submit.request.requestHeaders['Accept-Language'] = ['en;q=0.8']
}
The mentioned header will be added run time for each Rest Request Test step.
If you do not want to add any header you may just comment it out or add conditionally as well.
The whole above will just act as similar to(Pro) SubmitListener.beforeSubmit() and add the above code.