BUG: Polish translation warnings

[VincentTam]

Bug Report

Describe the bug

When I merged the recent updates into my test site (https://framagit.org/VincentTam/fish-demo), I observe some warning message about the plural form of a few Polish strings.

WARN 2021/03/07 07:29:14 Failed to get translated string for language "pl" and ID "reading_time": message "reading_time" has no plural form "few"
WARN 2021/03/07 07:29:14 Failed to get translated string for language "pl" and ID "reading_time": message "reading_time" has no plural form "many"
WARN 2021/03/07 07:29:14 Failed to get translated string for language "pl" and ID "reading_time": message "reading_time" has no plural form "few"
WARN 2021/03/07 07:29:14 Failed to get translated string for language "pl" and ID "categories": message "categories" has no plural form "few"
WARN 2021/03/07 07:29:14 Failed to get translated string for language "pl" and ID "reading_time": message "reading_time" has no plural form "few"
WARN 2021/03/07 07:29:14 Failed to get translated string for language "pl" and ID "categories": message "categories" has no plural form "few"
WARN 2021/03/07 07:29:26 Failed to get translated string for language "pl" and ID "reading_time": message "reading_time" has no plural form "many"
WARN 2021/03/07 07:29:26 Failed to get translated string for language "pl" and ID "categories": message "categories" has no plural form "few"

To Reproduce

I guess this issue applies to every blog with more than one Polish post.

How I get this warning?

1. Clone my repo at https://framagit.org/VincentTam/fish-demo.
2. Run hugo serve
3. See error.

Expected behavior

No warning message.

Screenshots

Environment

The client info seems to be irrelevant to this issue.

• Hugo Version: 0.80.0/extended
• Browser and Version: Firefox 86.0 (64-bit)
• Platform: Desktop
• Link to Project: https://framagit.org/VincentTam/fish-demo

Additional context

• Related: BUG: Categories didn't changed to Kategorie in PL language #184, Update site-sidebar.html (fix for translation of categories, issue #184) #192
• Cause: lack of the key few in items containing one.
  

  hugo-future-imperfect-slim/i18n/pl.toml

  Lines 25 to 31 in ef1cf98

  	[categories]
  	one = "Kategoria"
  	many = "Kategorie"
  	other = "Kategorie"
  	[tags]
  	one = "Tag"
  	other = "Tagi"

• My similar issue: Corrected Polish translation for "minute" victoriadrake/hugo-theme-introduction#174
• Unicode language plural rules is followed by Golang.