Just ideas:
-The 'Pan' can be difficult; if other users confirm, a tool&icon could be needed
-Gather the three grey upper icons under 'Settings'(Gear Icon)
-Give a direct access to the languages (as in the TA2 viewer)
-Set a darker grey to the unavailable icons; the 'isolate' icons could even be invisible when nothing is selected
-The 'isolate on layer' could be an option of of 'isolate'
-The function of the icon could be displayed under, or even replace the icon (Select; Isolate; Isolate on Layer; Hide; Frame All/Frame Selected, Cross Section)
-The upper left Cube icon could be simplified (remove the cube) and the 'X', '-X', ... replaced by 'TOP', 'BOTTOM', 'LEFT', 'RIGHT', 'FRONT', 'BACK' or by icons