I had a same problem with all the other plugin.(Polylang, qTranslate+, etc)
My default language wont be changed and I fixed it without any modification.
If you click to activate and allow the plugin very first time, the installed languages in the “Default Language / Order” are like below.
O Deutsch
0 English(checked – default language)
I solved the problem like below.
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1. disable all those languages at “Action”
2. add your default language(NL).
3. add rest of languages.