Although StudioSm gives an answer that works, the better way is of course to use the xml:lang attribute in your DITA files to determine the language settings in the documents. To do this you should look at the attribute in the root element and set your language as ContextRule in the TextFormatRules for that root element. There is a small difficulty to overcome, though. The EDD uses a colon in attribute references to allow pointing at an attribute of a parent element. The DITA xml:lang attribute is using the colon to indicate the XML namespace.
To overcome this issue, you should rename the xml:lang attribute in your DITA template's EDD to lang and add a read-write rule that translates between the xml:lang in the DITA file and lang in your FrameMaker EDD. It is a bit of work, but if you work in an environment where multiple languages are used (which is the majority of tech docs I guess), you will have to use this to allow translations with correct language settings. The only other option you would have is to create one set of templates for each different language - and you would have to quit FM, swap out the templates and restart FM every time you want to work on (or publish) DITA materials in another language.
So, if you are (going to) work in multiple languages, I suggest biting the bullet and doing it the proper way.