I believe you can set this in your .adp file, by calling a template with a src='' parameter. You could set what this is in your .tcl file, pass it to the adp file, and then it would be set dynamically.
Normally when there is a tcl and an adp file for a url, first the tcl file is processed and by default the corresponding adp file is then used for rendering the result. It is possible though to override this per request by including a call to 'ad_return_template alternative-file' at the bottom of the tcl page.
You can do the same in the default-master.tcl file:
...
set language "en"
ad_return_template "default-master-$language"
at the bottom of default-master.tcl will result in the templating code to look for a file called default-master-en.adp instead of the usual default-master.adp. Hope that helps - if not try to tell us more what you need to do.
How to find tcl procedures that do what you want: go to /api-doc/ of your local installation, type in a meaningful search term, e.g. 'parameter', and go through the results. In this particular case you have been looking for the proc parameter::set_value. ( http://openacs.org/api-doc/proc-view?proc=parameter%3a%3aset%5fvalue )
Note that procedures with :: in it are generally preferred because they use the more modern namespace feature and are propably newer.
An addition to the default-master hint above: I noticed that it won't work for pages under www/, unless you specify a full path like this:
