Implementing an own language negotiator

configure.zcml

<configure>
    <!-- Per request language overriding -->
    <adapter
        factory=".negotiate_language.NegotiateLanguage"
        for="* your.addon.interfaces.IAddonInstalled"
        provides="Products.PloneLanguageTool.interfaces.INegotiateLanguage"
      />
</configure>

negotiate_language.py

"""Language Negotiator to allow overriding language per request."""

from Products.PloneLanguageTool.interfaces import INegotiateLanguage
from Products.PloneLanguageTool.LanguageTool import NegotiateLanguage as NegotiateLanguageBase
from zope.interface import implements


class NegotiateLanguage(object):
    """Use default language negotiation, but allow overriding in certain requests."""
    implements(INegotiateLanguage)

    def __init__(self, site, request):
        """Setup the current language information."""

        base_negotiate = NegotiateLanguageBase(site, request)

        self.default_language = base_negotiate.default_language
        self.language_list = base_negotiate.language_list

        # Put here your custom logic to decide which language to use,
        # e.g.        
        forced_language = request.get('force_language') 
        # -- OR -- 
        # if request.get('force_default_language'):
        #     forced_language = default_language
        
        # Some sanity check...
        if forced_language and forced_language in self.language_list:
            self.language = forced_language
        else:
            self.language = base_negotiate.language