I know that it is not supported... but is there a way to change the connection string of the Power BI Analysis Services Connector to SSAS Tabular, so we can add some argument to force the use of a particular language? (Yes, we use translations with BIDS Helper...)
Unfortunately like you guessed, this is not possible today. You can't edit the connection string from the service to the AS connector or from the AS connector to the AS server.
If that is an important scenario for you, please go ahead and add it here as an idea (if it wasn't already there) for other users to vote on and for the team to add to the backlog.
Thanks for reaching out.