Création interface web sur back end existant

We are providing highly specialized low level technology: We are planning to open mid Mach an access to our proprietary back end in order for our prospects and clients to manage a development and benchmarking environment.
We want to focus on visual and user experience quality.
We have not decided to choose a development framework yet: reliable and relevant guidance will be welcome.
We need immediate availability for a delivery in 2 weeks.
Working in our offices might be effective even though it is not mandatory. We are based in 9th arrondissement of Paris, at walking distance of Gare du Nord.
Bilingual english is mandatory. French speaking is not required.