To install dictionaries on your eReader:

You can easily install Dictionaries on the Onyx Boox e-readers. The e-readers support, among other things, the StarDict format.
There are many free translation dictionaries to download that you can easily put on your e-reader.

Below are a few examples:

English-Dutch (+ Dutch-English)
German-Dutch ( + Dutch-German)

The procedure to install dictionaries on your Onyx Boox E-reader is as follows:

Create a folder 'Dicts' on a (micro) SD card.
Extract the .Zip file you downloaded and put all files in the folder 'Dicts' on the SD card.
Insert the (micro) SD card into your e-reader and turn on the e-reader.
For each dictionary a separate sup folder is created in the folder 'Dicts'!

Using dictionary:
Open an e-book and choose the word you want to translate by holding down the word until the whole word is selected.
A submenu appears and you choose 'Dictionary'.
The translation will then be given.
You can also type in other words to have them translated.