layersconfig?lang=undefined requests are back

I'm still seeing layersconfig?lang=undefined requests.

I think this is related to the [feature recenter code](https://github.com/geoadmin/mf-geoadmin3/blob/master/src/components/map/MapDirective.js#L42) which calls loadForTopic while there's no language defined yet.

There's another problem: loadForTopic no longer itself sets `currentTopic` in the gaLayers service. This means that calling loadForTopic may lead to an inconsistent state.

These are major issues requiring "prio 1" in my opinion.