How to set language

This plugin is almost perfect for me.
I have made the code so that it displays only the month on the timeline. (January, February etc.) But now I want this in translation, to be more specific, in Dutch.
My wordpress localization is already set to nl_NL but months stay in English.

Any help would be appreciated!

https://www.ads-software.com/extend/plugins/wp-jquery-timelinr/