The only way to achieve something like this would be to create a polygon mask which covers the globe but has a cutout for the boundary of Germany. By default the vector labels will still appear outside of Germany since they are rendered in a separate canvas above polygons. I create a sample and stored the mask polygon as GeoJSON. You can try the sample out here: https://rbrundritt.azurewebsites.net/Demos/BingMaps/GermanyMask/
You can download the polygon mask GeoJSON file I used here (note that it isn't super high resolution): https://rbrundritt.azurewebsites.net/Demos/BingMaps/GermanyMask/GermanyMaskPolygon.json
If you want the polygon to appear above the labels, you can put the map into lite mode. This will disable vector labels and labels will instead be static and a part of the base map tiles. This would clip labels at the edge of the boundary though and all shapes will be rendered above the labels which may not be ideal, however this also has an added performance boost. I have a modified version of the sample doing this here: https://rbrundritt.azurewebsites.net/Demos/BingMaps/GermanyMask/lite.html
Note, in case you missed it, Bing Maps is retiring: https://www.microsoft.com/en-us/maps/bing-maps You can find examples for polygon masks in Azure Maps here: https://samples.azuremaps.com/?search=mask In Azure Maps you can have the polygon mask render above labels.