Add option to set a locale

As mentioned in https://www.php.net/manual/en/function.basename.php both
basename() and dirname() are locale aware.
An incorrect locale can cause the page tree to behave in strange ways.

For example the structure

/über-uns
 |-index.md
 |-impressum.md

could lead to the page impressum not being a child of über-uns.

config/config.yml.template

@@ -7,6 +7,7 @@ base_url: ~                         # Pico will try to guess its base URL, if th
 rewrite_url: ~                      # A boolean (true or false) indicating whether URL rewriting is forced
 debug: ~                            # Set this to true to enable Pico's debug mode
 timezone: ~                         # Your PHP installation might require you to manually specify a timezone
+locale: ~                           # Your PHP installation might require you to manually specify a locale to use
 
 ##
 # Theme

lib/Pico.php

@@ -936,6 +936,7 @@ class Pico
             'rewrite_url' => null,
             'debug' => null,
             'timezone' => null,
+            'locale' => null,
             'theme' => 'default',
             'theme_config' => null,
             'theme_meta' => null,
@@ -974,6 +975,10 @@ class Pico
         }
         date_default_timezone_set($this->config['timezone']);
 
+        if ($this->config['locale'] !== null) {
+            setlocale(LC_ALL, $this->config['locale']);
+        }
+
         if (!$this->config['plugins_url']) {
             $this->config['plugins_url'] = $this->getUrlFromPath($this->getPluginsDir());
         } else {