[FX]
... // some code here
// Uniforms
...
float opacity = 1.0; // variable that controls opacity of the object

// contexts
context TRANSLUCENT
{
   VertexShader = compile GLSL VS_GENERAL;
   PixelShader = compile GLSL FS_TRANSLUCENT;
   
   ZWriteEnable = false;
   BlendMode = Blend;
}

... // some code here
[[FS_TRANSLUCENT]]
// =================================================================================================

uniform sampler2D albedoMap;
varying vec2 texCoords;
uniform float opacity;

void main( void )
{
   // gl_FragColor = texture2D( albedoMap, texCoords * vec2( 1, -1 ) ); // use this line to use transparency from the texture (png, tga with alpha channel)
   
   vec3 newCoords = vec3( texCoords, 0 );
   newCoords.t *= -1.0; // Flip texture vertically to match the GL coordinate system
   vec4 albedo = texture2D( albedoMap, newCoords.st );
   
   gl_FragColor = vec4( albedo.rgb, opacity ); // use this line to control transparency from material
}

 <Uniform name="opacity" a="0.5" />  // for 50% transparency