This issue has been discussed before but it has yet to be resolved by Inductive Automation.
See here: translate in expression tag.
I can say for sure that this is a problem on v7.8.2 (b2016030813), and v7.9.0 (b2016101208).
On the Translation Manager I’ve added the Key “Hello” with English (Alternate) se to “Hi”, and Spanish (Mexico) set to “Hola”.
I’ve created an Expression Tag with the following properties:
Data Type: String
Access Rights: Read/Write
Scan Class: Default
For the Expression I’ve tried all of the following, an all have failed miserably:
runScript(‘system.util.translate’, 0, ‘Hello’)