Use BCP 47 language format in output formats

* tta/C/convert/html_prepare_converter.c (html_setup_output),
tta/perl/Texinfo/Convert/Converter.pm (current_bcp47_locale),
tta/perl/Texinfo/Convert/DocBook.pm (conversion_output_begin)
(_convert), tta/perl/Texinfo/Convert/HTMLNonXS.pm (_setup_output),
tta/perl/Texinfo/Convert/LaTeX.pm (_informative_command_output)
(_latex_header): add current_bcp47_locale to get the BCP 47 language
locale in the current_lang_translations.  Output BCP 47 language
locales for output formats, as output formats use this language
specification format.

* tta/C/main/convert_utils.h: do not include translations.h in
convert_utils.h.  Update users.

* tta/C/main/translations.c (init_lang_translation): add const.