Updated info

When trying to install Bulgarian language pack from the Install Languages option inside Joomla (after successful install) the error is

Warning
Aborting language installation: Extension must have a title.
Package Install: There was an error installing an extension: site_bg-BG.zip

Error
Error installing package

So this means that

1. there is an error in the language pack
2. there is an error in the joomla install when it comes to finding the error strings

hmm looks like the lib language files are not loaded on that page?

any specific lib string has to be added in the installation language file.
we have already a bunch of them there
for example

; Framework strings necessary when no lang pack is available
JLIB_DATABASE_ERROR_CONNECT_MYSQL="Could not connect to MySQL."
JLIB_DATABASE_ERROR_DATABASE="A Database error occurred."
JLIB_DATABASE_ERROR_LOAD_DATABASE_DRIVER="Unable to load Database Driver: %s"
JLIB_ENVIRONMENT_SESSION_EXPIRED="Your session has expired, please reload the page."
JLIB_FILESYSTEM_ERROR_COPY_FAILED="Copy failed"
JLIB_FILESYSTEM_ERROR_PATH_IS_NOT_A_FOLDER_FILES="JFolder: :files: Path is not a folder. Path: %s"
JLIB_FORM_FIELD_INVALID="Invalid field: "
JLIB_FORM_VALIDATE_FIELD_INVALID="Invalid field: %s"
JLIB_FORM_VALIDATE_FIELD_REQUIRED="Field required: %s"
JLIB_INSTALLER_ERROR_FAIL_COPY_FILE="JInstaller: :Install: Failed to copy file %1$s to %2$s."
JLIB_INSTALLER_NOT_ERROR="If the error is related to the installation of TinyMCE language files it has no effect on the installation of the language(s). Some language packs created prior to Joomla! 3.2.0 may try to install separate TinyMCE language files. As these are now included in the core they no longer need to be installed."
JLIB_UTIL_ERROR_CONNECT_DATABASE="JDatabase: :getInstance: Could not connect to database <br />joomla.library: %1$s - %2$s"

etc.

as for bg-BG, will look at that pack as I already corrected a few of them yesterday.

For bg-BG (version 3.6.5.1), the issue is that , their install.xml name contains only unicode characters.
<name>Български</name>
As of 3.7.0, the should contain ascii

Do you want me to update the language file with the strings or will you

Closed see #16014