Hi Fabrice,
Unfortunately the editor does not support translucent materials with specularity at this release. It is being worked on currently and will be featured in a later build.
You can achieve the desired effect through a CubeMap in your material. To do this, import a HDRI texture to the editor; this is show the “reflection” image. Create a Texture Sample Parameter Cube in the material and then add your HDRI texture as the node’s texture. Create a Vector Transform and plug that into the ParamCube’s UV. Create a Reflection Vector and plug that into the Vector Transform. After this is set up, attach the ParamCube to BaseColor.
You will end up with a result like this:
I hope that this has helped to resolve the issue. If there is anything else that you need assistance with, please do not hesitate to ask.
Thank you,
Alexander