diff --git a/admin/tool/langimport/classes/locale.php b/admin/tool/langimport/classes/locale.php
new file mode 100644
index 00000000000..69c43b69cf0
--- /dev/null
+++ b/admin/tool/langimport/classes/locale.php
@@ -0,0 +1,84 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Helper class for the language import tool.
+ *
+ * @package    tool_langimport
+ * @copyright  2018 Université Rennes 2 {@link https://www.univ-rennes2.fr}
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+namespace tool_langimport;
+
+use coding_exception;
+
+defined('MOODLE_INTERNAL') || die;
+
+/**
+ * Helper class for the language import tool.
+ *
+ * @copyright  2018 Université Rennes 2 {@link https://www.univ-rennes2.fr}
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class locale {
+    /**
+     * Checks availability of locale on current operating system.
+     *
+     * @param string $langpackcode E.g.: en, es, fr, de.
+     * @return bool TRUE if the locale is available on OS.
+     * @throws coding_exception when $langpackcode parameter is a non-empty string.
+     */
+    public function check_locale_availability(string $langpackcode) : bool {
+        global $CFG;
+
+        if (empty($langpackcode)) {
+            throw new coding_exception('Invalid language pack code in \\'.__METHOD__.'() call, only non-empty string is allowed');
+        }
+
+        // Fetch the correct locale based on ostype.
+        if ($CFG->ostype === 'WINDOWS') {
+            $stringtofetch = 'localewin';
+        } else {
+            $stringtofetch = 'locale';
+        }
+
+        // Store current locale.
+        $currentlocale = $this->set_locale(LC_ALL, 0);
+
+        $locale = get_string_manager()->get_string($stringtofetch, 'langconfig', $a = null, $langpackcode);
+
+        // Try to set new locale.
+        $return = $this->set_locale(LC_ALL, $locale);
+
+        // Restore current locale.
+        $this->set_locale(LC_ALL, $currentlocale);
+
+        // If $return is not equal to false, it means that setlocale() succeed to change locale.
+        return $return !== false;
+    }
+
+    /**
+     * Wrap for the native PHP function setlocale().
+     *
+     * @param int $category Specifying the category of the functions affected by the locale setting.
+     * @param string $locale E.g.: en_AU.utf8, en_GB.utf8, es_ES.utf8, fr_FR.utf8, de_DE.utf8.
+     * @return string|false Returns the new current locale, or FALSE on error.
+     */
+    protected function set_locale(int $category = LC_ALL, string $locale = '0') {
+        return setlocale($category, $locale);
+    }
+}
diff --git a/admin/tool/langimport/index.php b/admin/tool/langimport/index.php
index 92a09dad241..1180abc09a8 100644
--- a/admin/tool/langimport/index.php
+++ b/admin/tool/langimport/index.php
@@ -109,9 +109,16 @@ echo $OUTPUT->header();
 echo $OUTPUT->heading(get_string('langimport', 'tool_langimport'));
 
 $installedlangs = get_string_manager()->get_list_of_translations(true);
+$locale = new \tool_langimport\locale();
 
+$missinglocales = '';
 $missingparents = array();
-foreach ($installedlangs as $installedlang => $unused) {
+foreach ($installedlangs as $installedlang => $langpackname) {
+    // Check locale availability.
+    if (!$locale->check_locale_availability($installedlang)) {
+        $missinglocales .= '<li>'.$langpackname.'</li>';
+    }
+
     $parent = get_parent_language($installedlang);
     if (empty($parent)) {
         continue;
@@ -121,6 +128,14 @@ foreach ($installedlangs as $installedlang => $unused) {
     }
 }
 
+if (!empty($missinglocales)) {
+    // There is at least one missing locale.
+    $a = new stdClass();
+    $a->globallocale = moodle_getlocale();
+    $a->missinglocales = $missinglocales;
+    $controller->errors[] = get_string('langunsupported', 'tool_langimport', $a);
+}
+
 if ($availablelangs = $controller->availablelangs) {
     $remote = true;
 } else {
diff --git a/admin/tool/langimport/lang/en/tool_langimport.php b/admin/tool/langimport/lang/en/tool_langimport.php
index faeb02cd5d6..27739eda343 100644
--- a/admin/tool/langimport/lang/en/tool_langimport.php
+++ b/admin/tool/langimport/lang/en/tool_langimport.php
@@ -37,6 +37,7 @@ $string['langpackupdateskipped'] = 'Update of \'{$a}\' language pack skipped';
 $string['langpackuptodate'] = 'Language pack \'{$a}\' is up-to-date';
 $string['langpackupdated'] = 'Language pack \'{$a}\' was successfully updated';
 $string['langpackupdatedevent'] = 'Language pack updated';
+$string['langunsupported'] = '<p>Your server does not seem to fully support the following languages:</p><ul>{$a->missinglocales}</ul><p>Instead, the global locale ({$a->globallocale}) will be used to format certain strings such as dates or numbers.</p>';
 $string['langupdatecomplete'] = 'Language pack update completed';
 $string['missingcfglangotherroot'] = 'Missing configuration value $CFG->langotherroot';
 $string['missinglangparent'] = 'Missing parent language <em>{$a->parent}</em> of <em>{$a->lang}</em>.';
diff --git a/admin/tool/langimport/tests/locale_test.php b/admin/tool/langimport/tests/locale_test.php
new file mode 100644
index 00000000000..4d1ffef94aa
--- /dev/null
+++ b/admin/tool/langimport/tests/locale_test.php
@@ -0,0 +1,71 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Tests for \tool_langimport\locale class.
+ *
+ * @package    tool_langimport
+ * @copyright  2018 Université Rennes 2 {@link https://www.univ-rennes2.fr}
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+defined('MOODLE_INTERNAL') || die();
+
+/**
+ * Tests for \tool_langimport\locale class.
+ *
+ * @copyright  2018 Université Rennes 2 {@link https://www.univ-rennes2.fr}
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class locale_testcase extends \advanced_testcase {
+    /**
+     * Test that \tool_langimport\locale::check_locale_availability() works as expected.
+     *
+     * @return void
+     */
+    public function test_check_locale_availability() {
+        // Create a mock of set_locale() method to simulate :
+        // - first setlocale() call which backup current locale
+        // - second setlocale() call which try to set new 'es' locale
+        // - third setlocale() call which restore locale.
+        $mock = $this->getMockBuilder(\tool_langimport\locale::class)
+            ->setMethods(['set_locale'])
+            ->getMock();
+        $mock->method('set_locale')->will($this->onConsecutiveCalls('en', 'es', 'en'));
+
+        // Test what happen when locale is available on system.
+        $result = $mock->check_locale_availability('en');
+        $this->assertTrue($result);
+
+        // Create a mock of set_locale() method to simulate :
+        // - first setlocale() call which backup current locale
+        // - second setlocale() call which fail to set new locale
+        // - third setlocale() call which restore locale.
+        $mock = $this->getMockBuilder(\tool_langimport\locale::class)
+            ->setMethods(['set_locale'])
+            ->getMock();
+        $mock->method('set_locale')->will($this->onConsecutiveCalls('en', false, 'en'));
+
+        // Test what happen when locale is not available on system.
+        $result = $mock->check_locale_availability('en');
+        $this->assertFalse($result);
+
+        // Test an invalid parameter.
+        $locale = new \tool_langimport\locale();
+        $this->expectException(coding_exception::class);
+        $locale->check_locale_availability('');
+    }
+}
diff --git a/lib/moodlelib.php b/lib/moodlelib.php
index 5ebe7afedec..3c44836fb0b 100644
--- a/lib/moodlelib.php
+++ b/lib/moodlelib.php
@@ -8010,6 +8010,28 @@ function moodle_major_version($fromdisk = false) {
 
 // MISCELLANEOUS.
 
+/**
+ * Gets the system locale
+ *
+ * @return string Retuns the current locale.
+ */
+function moodle_getlocale() {
+    global $CFG;
+
+    // Fetch the correct locale based on ostype.
+    if ($CFG->ostype == 'WINDOWS') {
+        $stringtofetch = 'localewin';
+    } else {
+        $stringtofetch = 'locale';
+    }
+
+    if (!empty($CFG->locale)) { // Override locale for all language packs.
+        return $CFG->locale;
+    }
+
+    return get_string($stringtofetch, 'langconfig');
+}
+
 /**
  * Sets the system locale
  *
@@ -8023,20 +8045,11 @@ function moodle_setlocale($locale='') {
 
     $oldlocale = $currentlocale;
 
-    // Fetch the correct locale based on ostype.
-    if ($CFG->ostype == 'WINDOWS') {
-        $stringtofetch = 'localewin';
-    } else {
-        $stringtofetch = 'locale';
-    }
-
     // The priority is the same as in get_string() - parameter, config, course, session, user, global language.
     if (!empty($locale)) {
         $currentlocale = $locale;
-    } else if (!empty($CFG->locale)) { // Override locale for all language packs.
-        $currentlocale = $CFG->locale;
     } else {
-        $currentlocale = get_string($stringtofetch, 'langconfig');
+        $currentlocale = moodle_getlocale();
     }
 
     // Do nothing if locale already set up.