Dutch is not officially supported by the game (you can see in Steam page languages section).
You can see english version of texts in “AoE2DE\resources\en\strings\key-value\key-value-strings-utf8.txt”. Modding info is in “key-value-modded-strings-utf8.txt” file in same folder. It is possible to translate them, but like you can see, there are a lot of texts and so this is really a lot of work.
It might be possible to create a program, which translates textual parts of string file to dutch using Google Translate API. You can’t translate whole file using API, because it has things, which should not be translated, like “<ORANGE>”, etc. Google Translate does not do perfect job though.
Maybe your wife could use this game for learning english. Would be helpful 