There is no reason whatsoever to think that the promotion of Esperanto preserves other ‘endangered’ languages. How would the use of Esperanto in Scotland preserve Scottish Gaelic, for instance?
Esperanto is a tool, not an end in itself. Nothing is more daft than slogans like ‘Esperanto brings about peace and friendship.’ People who have learnt Esperanto may support peace and friendship, but Esperanto is one tool among several in that cause. It would be mad to claim ‘cotton is the cause of red flags’ simply because you can make a red flag out of cotton material.
The issue for socialism is: if Esperanto is a tool, how can that tool be used in the service of socialism.