diff --git a/model/Config.php b/model/Config.php
index 4eb94366..53f2586e 100644
--- a/model/Config.php
+++ b/model/Config.php
@@ -218,7 +218,18 @@ final class Config {
      * @return string value of $PALANG[$var], parsed by sprintf
      */
     public static function lang_f($var, $value) {
-        return self::read_f('__LANG.' . $var, $value);
+        $all = self::read_array('__LANG');
+
+        $text = $all[$var] ?? '';
+
+        $newtext = sprintf($text, $value);
+
+        # check if sprintf changed something - if not, there are chances that $text didn't contain a %s
+        if ($text == $newtext) {
+            error_log("$var used via read_f, but nothing replaced (value $value)");
+        }
+
+        return $newtext;
     }
 
     /**
diff --git a/tests/ConfigTest.php b/tests/ConfigTest.php
new file mode 100644
index 00000000..33a84d62
--- /dev/null
+++ b/tests/ConfigTest.php
@@ -0,0 +1,33 @@
+<?php
+
+class ConfigTest extends \PHPUnit\Framework\TestCase {
+    public function setUp() : void {
+        $c = Config::getInstance();
+
+        $all = $c->getAll();
+
+        $all['xmlrpc_enabled'] = false;
+
+        $c->setAll($all);
+
+        parent::setUp();
+    }
+
+    public function testLangF() {
+        $x = Config::lang_f('must_be_numeric', 'foo@bar');
+
+        $this->assertEquals('foo@bar must be numeric', $x);
+    }
+
+    public function testLang() {
+        $x = Config::lang('must_be_numeric', 'foo@bar');
+
+        $this->assertEquals('%s must be numeric', $x);
+    }
+
+    public function testBool() {
+        $x = Config::bool('xmlrpc_enabled');
+
+        $this->assertFalse($x);
+    }
+}