use po file rather than Gajim code to handle plural form. see #6794

po/ru.po

@@ -6678,14 +6678,10 @@ msgstr "_Поиск по Web"
 msgid "Open as _Link"
 msgstr "Открыть как _ссылку"
 
-#: ../src/conversation_textview.py:1308
-msgid "Yesterday"
-msgstr "Вчера"
-
 #. %i is day in year (1-365)
 #: ../src/conversation_textview.py:1311
 #, python-format
-msgid "%(nb_days)i day ago"
+msgid "Yesterday"
 msgid_plural "%(nb_days)i days ago"
 msgstr[0] "%(nb_days)i день назад"
 msgstr[1] "%(nb_days)i  дня назад"

src/conversation_textview.py

@@ -1304,13 +1304,10 @@ class ConversationTextview(gobject.GObject):
                 int(timegm(tim)) / 86400
         if diff_day == 0:
             day_str = ''
-        elif diff_day == 1:
-            day_str = _('Yesterday')
         else:
             #%i is day in year (1-365)
-            day_str = i18n.ngettext('%(nb_days)i day ago',
-                '%(nb_days)i days ago', diff_day, {'nb_days': diff_day},
-                {'nb_days': diff_day})
+            day_str = i18n.ngettext('Yesterday', '%(nb_days)i days ago',
+                diff_day, replace_plural={'nb_days': diff_day})
         if day_str:
             format_ += day_str + ' '
         timestamp_str = gajim.config.get('time_stamp')