When creating a business rule and adding a message, the message is not properly translated if you switch to another language. This occurs even if you follow the steps outlined in the Use translated text documentation.
There is a known PRB for the issue: PRB719317
To resolve this issue, you can use a sys_ui_message and gs.getMessage as outlined below.
1) In the business rule record, ensure the Add message checkbox is not checked
2) Check the Advanced checkbox
3) In the Script field, add code similar to the following: gs.addInfoMessage( gs.getMessage( "YOUR_MESSAGE") );
4) Save the record
5) In the filter navigator type: sys_ui_message.list and press Enter
6) Click the New button to create a new message record
7) For the Key, enter the text you wish to have translated (this would be YOUR_MESSAGE from step 3)
8) For Language, select the target translation language
9) In the Message field, add the translated value that should appear
10) Save the record