lang: first steps for Russian

Taken from: https://translate.opnsense.org/

lang/Makefile

@@ -12,7 +12,7 @@ LOCALEDIR=	/usr/local/share/locale/%%LANG%%/LC_MESSAGES
 # stable
 LANGUAGES=	de_DE fr_FR ja_JP zh_CN
 # devel
-LANGUAGES+=	es_CO mn_MN nl_NL
+LANGUAGES+=	es_CO mn_MN nl_NL ru_RU
 
 TEMPLATE=	en_US
 INSTALL=

src/www/system_general.php

@@ -47,6 +47,7 @@ function get_locale_list()
     $locales['de_DE'] = gettext('German');
     $locales['ja_JP'] = gettext('Japanese');
     $locales['mn_MN'] = gettext('Mongolian');
+    $locales['ru_RU'] = gettext('Russian');
     $locales['es_CO'] = gettext('Spanish');
 
     return $locales;