Hide visible labels before translated

Related to issue #1927

[Test](http://mf-geoadmin3.dev.bgdi.ch/kan_hide_labels_bef_translated/src)

Actual: 
![wo-translate-cloak](https://cloud.githubusercontent.com/assets/8038575/5315101/8585d052-7c79-11e4-9c3e-6b20a72624f2.png)

Result: 
![w-translate-cloak](https://cloud.githubusercontent.com/assets/8038575/5315193/aa2a8212-7c7a-11e4-9d8e-a34f6f6c3970.png)

Seems not supported on IE. Still have to adapt on mobile.