Comment 7 for bug 1070210

I would also note that LibreOffice ignores not only alterations made with LC_* variable assignments but also manually compiled locale definition files with category keyword values differing from those LibreOffice considers right for that language.
I have slightly tuned ru_RU.utf8 locale definitions to use a point as a decimal separator, compiled it and set it as my current locale. LibreOffice keeps ignore locale settings with suggesting a comma as a decimal point.
$ locale
LANG=ru_RU.utf8
LC_CTYPE="ru_RU.utf8"
LC_NUMERIC="ru_RU.utf8"
LC_TIME="ru_RU.utf8"
LC_COLLATE="ru_RU.utf8"
LC_MONETARY="ru_RU.utf8"
LC_MESSAGES="ru_RU.utf8"
LC_PAPER="ru_RU.utf8"
LC_NAME="ru_RU.utf8"
LC_ADDRESS="ru_RU.utf8"
LC_TELEPHONE="ru_RU.utf8"
LC_MEASUREMENT="ru_RU.utf8"
LC_IDENTIFICATION="ru_RU.utf8"
LC_ALL=
$ locale -ck LC_NUMERIC
LC_NUMERIC
decimal_point="."
thousands_sep=" "
grouping=3;3
numeric-decimal-point-wc=46
numeric-thousands-sep-wc=8239
numeric-codeset="UTF-8"

Custom user-defined system locales (e.g. en_RU.utf8 - "Engilsh locale for Russia") also are not recognized by LibreOffice's "Language of locale setting" chooser - the option falls back to "Default - en".