The FMP 14 installation guide says:
"At the start of installation, you choose a language version for the FileMaker Pro installer. The default selection matches your operating system language. A Complete install will install only the language version you choose. You can install additional language versions by choosing Custom install, then selecting other language packs from the list of supported languages."
So at some point early installation process you are asked for the installed language.
Thank you, Bernhard.
I changed system language to English and installed again. Indeed was English (Engels:) default. With Custom Install I installed another language indeed (Dutch...) and now I can switch between languages.
A nice side effect, I could not use the FM help system, since Dutch was not installed (only English strange enough). Now I have English help in English and Nederlands in Dutch...