Your custom template has changed the original code in tpl_header.php, and in so doing they removed the functionality that normally would work if you simply edited the header.php language file as you had mentioned earlier.
The original code for displaying the logo in tpl_header.php is this:
Code:
zen_image($template->get_template_dir(HEADER_LOGO_IMAGE, DIR_WS_TEMPLATE, $current_page_base,'images'). '/' . HEADER_LOGO_IMAGE, HEADER_ALT_TEXT)
Replace yours with that, and then your changes to the header.php language file will take effect normally.
Remember: every addon you install may or may not have had original features removed by whoever packaged the addon ... this could be because they didn't want the feature and didn't choose to turn it off via a built-in method, or they wanted to make it work another way. Some addons are built short-sighted simply to meet an immediate need, and others are built for optimal benefit to all who might choose to use them. Your mileage may vary with each. It's a chance you take.