After a long time looking for one-click language switch (,, i18n.module) and not being satisfied with the solutions, i come up with my own. (I should say that i'm not a php expert and if you like, use the below codes at your own risk.)

here is an example that uses sessions:
i did this for my site that uses English and Turkish translations, and it works. (for translations you must activate locale.module and add languages you will use.)
you may change the language codes 'tr' and 'en' or add more for your needs.

firstly, we have to hack> locale_initialize() function

function locale_initialize() {
  global $user;

  //Lines added - begin
  if ($_GET['mylang']=='tr' || $_GET['mylang']=='en') {// allow only tr and en tags. (change for your needs)
    return $_SESSION['mylang'] = $_GET['mylang'];
  if ($_SESSION['mylang']) return $_SESSION['mylang'];
  //Lines added - end

  if (function_exists('i18n_get_lang')) {

now what we should do is to send the variable 'mylang' with values 'en' or 'tr' (or what else you defined in locale_initialize) by GET method.
And also we should conserve the page path. we will use the $_GET['q'] variable for this.

BLOCK choice:
go to block settings and add this block:

echo l('English', $_GET['q'].'&mylang=en'); // link to /path&mylang=en
echo '
'; echo l('Turkish', $_GET['q'].'&mylang=tr'); // link to /path&mylang=tr

THEME choice:(only tested with xtemplate engine)
In themes you should use {PHP._GET.q} instead of $_GET['q']
Add this code to anywhere in your template file (here i used images instead of text links):

<a href="/?q={PHP._GET.q}&mylang=tr"><img src="/files/pictures/flags/tr.png"></a>
<a href="/?q={PHP._GET.q}&mylang=en"><img src="/files/pictures/flags/en.png"></a>

here the problem is the part "?q=". since you can't use the link function as in the block choice, if you use clean urls you must delete "?q=" from href attribute.
I wonder if there is any alternative that is independent of clean url usage.???
I also wonder if there is any alternative for one-click language switch wihout hacking core???