header.html inside your theme is actually parsed as PHP, so you can do whatever you like inside there, and access any of the standard moodle library functions. And if you need that level of complication, you could create some other PHP files (e.g. lib.php) inside your theme folder, and include them. And if you need other lang strings, you can create a new language file lang/en_utf8_local/my_theme_strings.php, and get those strings using get_string('stringkey', 'my_theme_strings');
Custom theme needs language support
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