Multi-select picklist field in Salesforce cannot be translated solely with Dynamo Translation control in Template Builder. For multiple selections which give collection value instead of a string value, it need to have a logic to go through each value and translate them using Translation control.
In step ‘Compose – Start’ in Main Flow, below the setTranslation command, enter the commands mentioned in the screenshot below. The sample code uses the multi-select picklist field from Opportunity, so please change the field in the query into the field and object where that multi-select field exists. The point is to query the multi-select field out first, apply translation logic to it in the Main Flow and use the field in the document body.
Afterwards use the variable, in this case is ‘translated’, in where you need the info to display in the document.