Localized read count for threads shows "1 read" for any number of reads ending with 1

Sounds like there is a problem with your languages format_plural function. The only references I could find:
http://api.drupal.org/api/function/format_plural/6
http://api.drupal.org/api/function/locale_get_plural/6
#68506: Misleading documentation and usage of format_plural() function
#30876: Bug with plural forms

You've imported a .po file with a broken plural formula. Import a .po file with a good plural formula, and this issue should be solved.

This is a really annoing issue for languages different from english...
Translators should know that they MUST use @count in singular

For example russian have 3 plural forms:

"Plural-Forms: nplurals=3; plural=((((n%10)==1)&&((n%100)!=11))?(0):(((((n%10)>=2)&&((n%10)<=4))&&(((n%100)<10)||((n%100)>=20)))?(1):2));\n"

msgid "1 comment"
msgid_plural "@count comments"
msgstr[0] "@count комментарий" // for 1, 21, 31 and others
msgstr[1] "@count комментария"
msgstr[2] "@count[2] комментариев"

Also JS Drupal.formatPlural() translation is totally broken #504506: Drupal.formatPlural incorrectly handle complex plural rules
EDIT: this is a different issue - #532512: Plural string storage is broken, editing UI is missing

There are three possible explanations for this issue:

• Either the plural formula of the language in question is broken (unlikely), or
• The source .po file of the translation doesn't follow the language plural convention (as in the example @andypost displayed, if you imagine that the first line reads 1 комментарий instead of @count комментарий), or
• The user tries to translate an auto-detected plural form from the UI, which is totally broken (#532512: Plural string storage is broken, editing UI is missing)

The two first are not related to core (and should be reported in the language-specific issue queue). We already have an issue for the third one.

=> Won't fix.