Only english as blocks language

https://github.com/UCL-INGI/INGInious-problems-blockly/blob/c018f863a210315eceff3c43dcbdaa1533c8e130/inginious-problems-blockly/templates/box_blockly.html#L108

By now, the blocks are in english and there is no workaround to change their language.
It would be very nice to be able to choose the blocks language, either by the task writer or the user itself with a `` box.
Some of the Blockly demos use such a box and are able to dynamically change the language (see [Blockly Demo](https://blockly-demo.appspot.com/static/demos/code/index.html) and [Corresponding code](https://github.com/google/blockly/tree/7ba9e5eac1b1f45cc7e0cb774ecd89af8fb07dd7/demos/code) )