Prerequisites:

• In a running Moodle instance, customize the "core_admin/sitemaintenancetitle" language string of the english language pack within the language customization GUI.
• Open your terminal and run this command:

  php admin/tool/customlang/cli/export.php --lang=en --target=/tmp/en_local

• Afterwards, there is a zip file at /tmp/en_local[[exportzipfilename]] (which is a bug reported at MDL-75970, just to be precise here). Rename this file to /tmp/en_local.zip for further procedure.

Steps to reproduce:

• Get a fresh Moodle installation where you are sure that nobody has made any language customizations yet.
• In your terminal, run this command in this fresh Moodle installation:

  php admin/tool/customlang/cli/import.php --lang=en --source=/tmp/en_local.zip --checkin --mode=all

Expected result:

• In your terminal, you should see this output:

  == Files to import into en ==

  /tmp/en_local.zip

  == Import files string (mode all) ==

  String core_admin/sitemaintenancetitle updated successfully.

  Save changes to the language pack

• The modified string takes effect in the Moodle GUI.

Actual result:

• In your terminal, you see this output:

  == Files to import into en ==

  /tmp/en_local.zip

  == Import files string (mode all) ==

  Missing component core_admin.

  Save changes to the language pack

• The modified string does not take effect in the Moodle GUI.

Analysis:

• The CLI import tool seems not to do the initial language pack checkout which is necessary before the first language string customization.
• In the GUI, if you go to Site administration -> Language -> Language customization -> Pick "English", this initial checkout happens automatically before you can use the customization GUI for the first time. As a workaround for this issue, you can do the initial checkout in the GUI and run the CLI importer afterwards.