Any (Linux/UNIX) stack works. We're not big enough to be doing much training language-wise so you'd have to be excited to learn more Python or JavaScript if you don't know those well.

We use ElasticSearch too so that's another technology that you'd have to be interested in.

On a side note, we use Amazon Aurora (MySQL 5.6) for the core data but most interactions are with ElasticSearch and Redis. MySQL is just our definitive datastore.