There is nothing specific to Tableau Extensions which would allow for what you are trying to regarding localization. Here is a KB article regarding - How to Localize Content in Tableau | Tableau Software. You could also consider a sheet selector - https://onlinehelp.tableau.com/current/pro/desktop/en-us/help.htm#dashboards_sheet_selector.html and have a different sheet for the various languages you want to support. Finally, there is a 3rd party extension Narratives for Tableau - Powered by Narrative Science which creates automated explanations of visualizations created in Tableau. Perhaps, their technology can employ localization to provide that automated explanation.
We are in the process of developing a Tableau Translation Application that will be finished in a couple of weeks if you still have an interest?
As Chaitanya noted, we have a lot of Tableau Users asking for this. I'm trying to keep track of similar requests on a new Internationalization Community Space in the Community Forums that will be made public any day now.
I'm so excited that you are developing a Dashboard Extension for this! Please keep me posted! I would love to add a post about this to the new Internationalization Space once you are ready to launch and the space is made public.
Please vote on this idea. It really helps me to push for new feature development.
Senior Globalization Engineer