Fixes #10599 / add note to escapeshellarg() removes non ascii chars

Documentation/Manual.rst

@@ -127,6 +127,14 @@ If there are problems with converting/downloading FE_GROUP protected pages, chec
     and so on! Typically some of them fails to load and wkhtml stops running! Verify the correct loading of all elements
     by calling the site via a regular browser and bypassing any browser cache (Ctrl F5).
+.. note::
+
+   On Ubuntu, Apache is started by default with `LANG=C`. This is true even when the OS default locale is set to `en_US.UTF-8`.
+   Furthermore, all child processes of Apache will inherit `LANG=C`. Some PHP functions (like 'escapeshellarg()')
+   or `wkhtml` will strip all non-ASCII characters (e.g. commandline arguments).
+
+   Let Apache run with the system locale: `/etc/apache/envvars`, activate the line `. /etc/default/locale` and restart Apache.
+
 Checklist wkhtml problems
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