Update en_US.lang

client/languages/en_US.lang

@@ -31,6 +31,8 @@ module.visual.setting.fixed_width.name=Fixed Width
 module.visual.setting.fixed_width.description=What width should it be.
 module.visual.setting.precision.name=Precision
 module.visual.setting.precision.description=The precision of the value.
+module.visual.setting.opacity.name=Opacity
+module.visual.setting.opacity.description=The opacity (how transparent it is).
 module.visual.setting.corner_roundness.name=Rounded Corner Radius
 module.visual.setting.corner_roundness.description=The size of the rounded part of corners.
 module.visual.setting.corner_quality.name=Rounded Corner Quality