In general, I suggest you read the Kirby documentation, it’s a bit much to answer such a general question here in the forum.
Basically, a template is called automatically depending on the file name of the text files in your content folder, e.g. if your text file is test.txt, Kirby will look for a test.php template. If the template does not exist, Kirby will fall back to the default.php template.
As regards the multi-lang feature, it is not enough to enable the languages in your config.php. You also need to rename your text files for each language, e.g. test.en.txt for English, test.fr.txt for French, etc. You also have to remove all text files without any language code extension, otherwise it will not work. That information is still missing from the docs, but we are working on it.
This is the place where you need to add your template names. site/blueprints/site.php defines the allowed templates for the root directory. If you want to use your templates as children of other templates, you have to change this setting in the corresponding blueprint file, too.
If you don’t want any restrictions at all and use all templates everywhere, simply change the line (in every blueprint file) to this:
Maybe this is a starting point for you. Don’t hesitate to ask, if you have more questions. There are a lot of nice people here around who will try to help you with your Kirby adventure.
If you want to make changes to the menu: There is probably a snippet in the snippets folder called “menu.php” or similar that contains the menu. So if you want to make changes in the menu, that is probably the file you want to look into.
A typical menu in Kirby will look like this:
// get main menu items, i.e. all visible page (folders with a prepended number)
$items = $pages->visible();
// check if there are any items
if($items->count() > 0):
//loop through the items
<?php foreach($items as $item): ?>
<li><a<?php ecco($item->isOpen(), ' class="active"') ?> href="<?php echo $item->url() ?>"><?php echo html($item->title()) ?></a></li>
<?php endforeach ?>
<?php endif ?>
For more information on how to create menus with submenus, check out this blog post
Without knowing the way the theme is coded, though, I can’t say for sure.