Well for AWS Lambda, they offer three languages: Java, Javascript, and Python. Even then, its really just running in a container and you have access to anything in their standard Linux AMI and whatever else you include with your code, so you could really use almost any language you want. I'm using wkhtml2pdf with my Python code which is a c++ executable. Also, although the Python is 2.7, you can easily use the existing Python 3 environment or include your own.
As far as lock in, it is trivial to utilize the same code on your own servers. I do that for my Python code for development using a simple Cherrypy setup in less than 100 lines of code.
As far as lock in, it is trivial to utilize the same code on your own servers. I do that for my Python code for development using a simple Cherrypy setup in less than 100 lines of code.