Hi arangoneo,
ArangoDB is currently on version 2.8 with version 3.0 being in active development, which will include major changes to the Foxx framework. However I don't believe that these changes are relevant to your question.
Foxx is ArangoDB's framework for writing self-hosted microservices using JavaScript. These services can be exposed by using ArangoDB's HTTP API but are deployed inside the database itself and run in the same JavaScript engine that is also used to handle AQL user functions and such. An example for a Foxx service is the web interface of ArangoDB itself, which provides not just an HTTP API but also some static assets.
That said, the language you use to write code that interfaces with the HTTP API (whether that is ArangoDB's native API or your own Foxx services) is entirely up to you. There are no unique advantages of one over the other. You can even make the HTTP API of your Foxx services look like whatever you want (just like an HTTP API written in Express).
Based on my personal experience however, I would suggest you try just skipping the server app (i.e. node or PHP) entirely and implement that logic as a Foxx service and have your client talk to it directly. You can still put something else in between if you find the need, but it may be easier to start this way because there are fewer moving parts.
BTW, Angular, Ember and Backbone are client-side front-end frameworks. They run in the browser, not in Node (which runs on the server). Kraken, Express, Sails and Hapi are Node frameworks. Foxx is to ArangoDB as Express is to Node.
Cheers,
Alan