Using LANGUAGE_TOOL.LANGUAGE to specify language of translated strings in TAL template -what makes this work?

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Thanks for the in-depth answer @dieter

We're using Plone 5.0.7 with plone.app.multilingual 5.0.3

The folder structure is explained here GitHub - plone/plone.app.multilingual: Plone Multilingual Content Add-on

The "active" language negotiation itself is not handled by plone.app.multilingual but by the plone.i18n.negotiator module. I should have paid more attention to the introduction section of https://docs.plone.org/develop/plone/i18n/language.html - namely: "Language is negotiated at the beginning of the page view."

Also noticing there is likely an error on that page: a reference to Products.PloneLanguageTool. It seems the tool was made part of plone.i18n in version 3.0.0 (2015-03-26).

I think I may understand why in my workaround all translated strings were in the language of the first document. Looking here: plone.i18n/plone/i18n/utility.py at master · plone/plone.i18n · GitHub

binding = request.get('LANGUAGE_TOOL', None)
if not isinstance(binding, LanguageBinding):
    # Not bound -> bind
    setLanguageBinding(request)
binding = request.get('LANGUAGE_TOOL')

This creates a new language binding when called the first time around...