How can I convert Evernote's notes to standard TXT files?

It is possible using Simplenote.

Download a Simplenote client that supports importing from Evernote and also supports exporting to text files. Here's a list of Simplenote clients to choose from.

Justnotes, for example, does that.

Here's a walkthrough. You'll need the Evernote desktop app. from http://www.documentsnap.com/how-to-export-your-data-out-of-evernote/

In the Notebooks section, right click on All notebooks
Choose Export Notes…
When the popup appears, in the Save As field give the export a name.  That will be the name of the folder (if you choose HTML) or the file (if you choose Evernote XML)
Choose the folder you want to save your export in
Decide whether you want to export as HTML or Evernote XML (read on to find out which one you want to choose)

UPDATE:

Based on OPs comments that existing export functions do not work, then the short answer mid no. By itself, without external apps, there is no way to export Evernote notes directly to text.

In order to get txt files, then data will have to be exported to a supported format such as HTML or XML and then converted to text using a 3rd party application.

There's a rich API you can use to get access to the data. See for example https://github.com/dvj/clevernote which uses the api to provide a command line interface to evernote. It converts the evernote markup to markdown, which is easy on the eyes, but still maintains the overall note structure.

I think there's a java implementation with similar goals.