After a bit of research, I suspect that this has nothing to do with the po files. Since they're only for the TimeMapper website and not for the resulting embeddable TimeMap.
The Timeline is this one: https://github.com/NUKnightLab/TimelineJS3
They have an option for language, but you have to specify that in the configuration which you don't have access to as an end user.
TimeMapper has a
tmconfig which has a
timelineJSOptions field, but it does not get populated as far as I can tell.
It seems that this should be created and added to the creation view.
Starting point would be here:
You can add a new field and try to add a
I do not have the time right now. Otherwise, I would help, but maybe someone with more knowledge of TimeMapper could help.