I came to conclusion, that the best way "to create language pack for Joomla! 1.5 extension" is the third way

:
to create "dummy plugin" package, which purpose is to give user ability:
- to install and uninstall language files for "real" (parent) extension, using Joomla! installer;
- to see, which languages of extension are installed (each language has it's own line in the list of plugins);
- to see the version of each language pack (version of language pack is version of extension, which language files were translated) - in the "Extension Manager->Plugins".
I had to make only one change in my extension: I've got rid of 'site' language files (I moved all localizable strings to 'administrator' language files), because plugin installer can't install 'site' language files
- but I've got another thing as a result: now I have only
one language pack file for each language
So, install.xml file is still very simple:
Code:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE install SYSTEM "http://dev.joomla.org/xml/1.5/plugin-install.dtd">
<install type="plugin" group="content" version="1.5.0">
<name>yvCommentLanguagePack_da-DK</name>
<creationDate>2008-03-23</creationDate>
<author>Yuri Volkov</author>
<authorEmail>Email -> Visit the website:</authorEmail>
<authorUrl>http://yurivolkov.com/Joomla/yvComment/index_en.html</authorUrl>
<copyright>(c) 2007-2008 yvolk (Yuri Volkov), http://yurivolkov.com. All rights reserved.</copyright>
<license>GPL</license>
<version>1.6.6</version>
<description>
This is Danish language pack for yvComment extension.
Don't enable this plugin.
See yvComment's Homepage [url]http://yurivolkov.com/Joomla/yvComment/index_en.html[/url] for more information.
</description>
<files>
<filename plugin="yvCommentLanguagePack_da-DK">yvCommentLanguagePack_da-DK.php</filename>
</files>
<languages>
<language tag="da-DK">da-DK.plg_content_yvcomment.ini</language>
<language tag="da-DK">da-DK.com_yvcomment.ini</language>
</languages>
</install>
As you may see in the code above, I had to add only one (dummy) plugin file to the package: yvCommentLanguagePack_da-DK.php
-It is required by Joomla! installer.
So, maybe this is good solution for the time being (until Joomla! will have the same support for "language" package as it has for "plugin" package).
See full package attached.