Steps to reproduce the issue

1. Enable SEO
2. Multi language site set up

3. use URL like this to check the proper functionality of the language:
   index.php?option=com_ajax&module=virtuemart_bohcatfilter&format=raw&nosef=1&lang=fr&ict=36

4. we are using:
   \modules\mod_virtuemart_bohcatfilter\helper.php

class modVirtuemartBohcatfilterHelper {
 public static function getAjax() {
   $lang = JFactory::getLanguage();
   $language_tag = $lang->getTag();
   var_dump($language_tag); die(); 
 }
}

1. you can see that the language_tag was IMPROPERLY SET to joomla's default language regarless of the configuration of the language filter plugin and the current value of the language cookie

2. FIX:

we found that altering line:

274: else
to
274: if (empty($lang_code))

fixes the problem.

More lines:

.... 
// We have found our language and the first part of our URL is the language prefix
                if ($found)
                {
                    array_shift($parts);
                    $uri->setPath(implode('/', $parts));
                }
            }
        }
        if (empty($lang_code))
        {
            // We are not in SEF mode
            $lang = $uri->getVar('lang');

            if (isset($this->sefs[$lang]))
            {
                // We found our language
                $found = true;
                $lang_code = $this->sefs[$lang]->lang_code;
            }
        }

in \plugins\system\languagefilter\languagefilter.php

JOOMLA 3.4.0

Furthermore the plugin SHOULD NOT DO A REDIRECT from within an Ajax context because it can easily redirect the user to the Ajax page for some old Chrome, Firefox or IE browsers. In short, there must not be any redirects from within the ajax context.

Before doing any redirect there can be a check done:

$doc = JFactory::getDocument(); 
$class = class_name($doc); 
$class = strtoupper($class); 
if ($class === 'JDOCUMENTHTML') ... 

Expected result

1. redirect to /default-lang/rest of the path...
2. wrong language detected from the URL

Ideal result:
1. No redirect to SEF from ajax context
2. Proper language detection

Actual result

System information (as much as possible)

Additional comments