first release

Framework/Translate.php

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+<?php
+
+namespace LavoWeb\TranslationInherit\Framework;
+
+use Magento\Framework\App\Config\ScopeConfigInterface;
+use Magento\Framework\App\ObjectManager;
+
+class Translate extends \Magento\Framework\Translate
+{
+    const PARENT_LOCALE_PATH = 'general/locale/parent_code';
+
+    /**
+     * Initialize translation data
+     * @override If parent locale is set in configuration, we load that translations first
+     *
+     * @param string|null $area
+     * @param bool $forceReload
+     * @return $this
+     * @throws \Magento\Framework\Exception\LocalizedException
+     */
+    public function loadData($area = null, $forceReload = false)
+    {
+        $this->_data = [];
+        if ($area === null) {
+            $area = $this->_appState->getAreaCode();
+        }
+        $this->setConfig(
+            [
+                self::CONFIG_AREA_KEY => $area,
+            ]
+        );
+
+        if (!$forceReload) {
+            $data = $this->_loadCache();
+            if (false !== $data) {
+                $this->_data = $data;
+                return $this;
+            }
+        }
+
+        // Begin override
+        $parentLocale = $this->getParentLocale();
+        $currentLocale = $this->getLocale();
+        if ($parentLocale != '' && $parentLocale != $currentLocale) {
+            $this->setLocale($parentLocale);
+
+            $this->_loadModuleTranslation();
+            $this->_loadPackTranslation();
+            $this->_loadThemeTranslation();
+            $this->_loadDbTranslation();
+
+            $this->setLocale($currentLocale);
+        }
+        // End override
+
+        $this->_loadModuleTranslation();
+        $this->_loadPackTranslation();
+        $this->_loadThemeTranslation();
+        $this->_loadDbTranslation();
+
+        if (!$forceReload) {
+            $this->_saveCache();
+        }
+
+        return $this;
+    }
+
+    /**
+     * Get parent locale
+     *
+     * @return string
+     */
+    protected function getParentLocale()
+    {
+        $config = ObjectManager::getInstance()->get(ScopeConfigInterface::class);
+        return $config->getValue(
+            self::PARENT_LOCALE_PATH,
+            \Magento\Store\Model\ScopeInterface::SCOPE_STORE
+        );
+    }
+}

Readme.md

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+# Translation Inherit
+
+Translation inherit/locale fallback for Magento 2
+
+## Installation
+
+```
+composer require lavoweb/translationinherit
+php bin/magento setup:upgrade
+```
+
+## Configuration
+
+Stores => Configuration => General => Locale Options => Parent Locale
+
+If parent locale is set and haven't the same value than current locale, they'll be merged.

composer.json

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+{
+    "name": "lavoweb/translationinherit",
+    "description": "Translation inherit/locale fallback for Magento 2",
+    "require": {
+        "magento/magento-composer-installer": "*"
+    },
+    "type": "magento2-module",
+    "version": "0.1.0",
+    "license": [
+
+    ],
+    "autoload": {
+        "files": [
+            "registration.php"
+        ],
+        "psr-4": {
+            "LavoWeb\\TranslationInherit\\": ""
+        }
+    },
+    "extra": {
+        "map": [
+            [
+                "*",
+                "LavoWeb/TranslationInherit"
+            ]
+        ]
+    }
+}

etc/adminhtml/system.xml

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+<?xml version="1.0"?>
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
+    <system>
+        <section id="general">
+            <group id="locale">
+                <field id="parent_code" translate="label" type="select" sortOrder="5" showInDefault="1" showInWebsite="1" showInStore="1">
+                    <label>Parent Locale</label>
+                    <source_model>Magento\Config\Model\Config\Source\Locale</source_model>
+                </field>
+            </group>
+        </section>
+    </system>
+</config>

etc/di.xml

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+<?xml version="1.0"?>
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
+    <preference for="Magento\Framework\Translate" type="LavoWeb\TranslationInherit\Framework\Translate" />
+</config>

etc/module.xml

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+<?xml version="1.0"?>
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+        xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
+    <module name="LavoWeb_TranslationInherit" setup_version="0.1.0" />
+</config>

registration.php

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+<?php
+
+\Magento\Framework\Component\ComponentRegistrar::register(
+    \Magento\Framework\Component\ComponentRegistrar::MODULE,
+    'LavoWeb_TranslationInherit',
+    __DIR__
+);