Foremost, thanks you tghuvr for all your work on en_GB localization, thus contributing to solve this problem.

Things like one you mentioned is proof that project of this size badly need lot of development effort to become and stay relevant – nothing not resolvable with enough manpower, which is needed to attract.

Regarding documentation, situation is improving:

http://glpi-install.readthedocs.io

https://readthedocs.org/projects/glpi-plugins

https://readthedocs.org/projects/glpi-developer-documentation/

this one is in process of rewrite, which is done in French

https://glpi-user-documentation.readthedocs.io

Regarding en_GB translation, not sure if non UK based, English (native or second language speaking ones) users will even notice, that there is possible to switch to it. They probably open application with "English" from source code (thus written by non-native speakers), they do not understand, what is application telling them and will end it using.

Nevertheless, in favor of en_uk speaking users, explored situation about localization for them, and result is:

en_GB as a source language (i.e. done by non-native speakers only):

https://www.transifex.com/tsmr/GLPI_accounts/
https://www.transifex.com/tsmr/GLPI_additionalalerts/
https://www.transifex.com/tsmr/GLPI_badges
https://www.transifex.com/pluginsGLPI/credit/
https://www.transifex.com/tsmr/GLPI_mydashboard
https://www.transifex.com/tsmr/GLPI_shellcommands
https://www.transifex.com/tsmr/GLPI_consumables
https://www.transifex.com/tsmr/GLPI_typology
https://www.transifex.com/tsmr/GLPI_tasklists
https://www.transifex.com/teclib/glpi-project-plugin-mreporting
https://www.transifex.com/tsmr/GLPI_moreticket
https://github.com/tomolimo/processmaker/tree/master/locales
https://www.transifex.com/tsmr/GLPI_timelineticket
https://www.transifex.com/teclib/glpi-plugin-used-items-export
https://www.transifex.com/tsmr/GLPI_financialreports
https://www.transifex.com/tsmr/GLPI_manufacturersimports
https://www.transifex.com/tsmr/GLPI_positions
https://www.transifex.com/tsmr/GLPI_resources
https://www.transifex.com/tsmr/GLPI_printercounters
(but is completely localized into en_US)
https://www.transifex.com/tsmr/GLPI_addressing
https://www.transifex.com/tsmr/GLPI_satisfaction
https://www.transifex.com/tsmr/GLPI_environment
https://www.transifex.com/tsmr/GLPI_databases
https://www.transifex.com/tsmr/GLPI_domains
(but is completely localized into en_US)
https://www.transifex.com/tsmr/GLPI_webapplications
https://www.transifex.com/teclib/glpi-plugin-mantisbt

Without en_GB localization altogether (similar to previous):

https://plmlab.math.cnrs.fr/sylvain.allemand/reservation/tree/master/locales
https://www.transifex.com/tsmr/GLPI_resources
https://www.transifex.com/teclib/glpi-project-plugin-escalade
(but is completely localized into en_US)
https://github.com/edgardmessias/browsernotification/tree/master/locales
https://www.transifex.com/teclib/glpi-plugin-uninstall
https://www.transifex.com/teclib/glpi-plugin-itop
https://www.transifex.com/teclib/glpi-plugin-datainjection
https://www.transifex.com/teclib/glpi-plugin-itilcategorygroups
https://www.transifex.com/teclib/glpi-project-plugin-genericobject

Translation opened, but without contributors or incomplete:

https://www.transifex.com/ddurieux/FusionInventory
(this one is really important, without it and alternatively OCS-NG, inventory must be done manually)
https://www.transifex.com/teclib/glpi-plugin-news
https://www.transifex.com/yllen/glpi-project-plugin-behaviors
https://www.transifex.com/teclib/glpi-plugin-plugin-fields
(but is almost completely localized into en_US)
https://www.transifex.com/yllen/GLPI_ocsinventoryng/
(the same as for FusionInventory)
https://www.transifex.com/yllen/glpi-project-plugin-pdf
https://www.transifex.com/yllen/glpi-project-plugin-geststock

en_GB translation almost done:

https://www.transifex.com/yllen/glpi-project-archires-plugin/languages/
https://www.transifex.com/yllen/GLPI_reports
https://www.transifex.com/yllen/glpi-project-plugin-appliance
https://www.transifex.com/teclib/glpi-plugin-order
https://www.transifex.com/teclib/glpi-project-plugin-formcreator

en_GB translation done (was reviewed?):
https://www.transifex.com/teclib/glpi-plugin-tag3
(and is completely localized into en_US)
https://www.transifex.com/yllen/GLPI_webservices
https://www.transifex.com/teclib/glpi-plugin-xivo
(and is completely localized into en_US)

I do know, it is long list thus lot of work (know it firsthand from hundreds hours of work to localize all into Czech), but it would be great to have proper translations into en_GB (and en_US too) to make GLPI sustainable - now GLPI and its plugins are done by very few or even solo developers, so not enough resources for substantial changes into code base and bus factor (https://en.wikipedia.org/wiki/Bus_factor) is pretty bad too (continuity is important factor for decision if deploy such product in production environment)…