Replace languages.inc globals with I18n\Languages consts (#1121)

The include/languages.inc file was not removed as it's used in other
repositories. It should be removed after migration.

Replace languages.inc globals with I18n\Languages consts (#1121)

The include/languages.inc file was not removed as it's used in other
repositories. It should be removed after migration.

Tests were added to ensure that the global variables and the constants
are in sync with each other.

Signed-off-by: Maurício Meneghini Fauth <mauricio@mfauth.net>

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