Description

This PR introduces support for the Polish locale and refactors the duration calculation logic to support languages with complex pluralization rules.

The Problem

Previously, RenderCV utilized a binary pluralization logic (singular if n=1, else plural). This is insufficient for languages like Polish, where nouns decline into three or more forms depending on the numeral:

• One (Singular): 1 miesiąc
• Few (Paucal): 2 miesiące, 22 miesiące (ends in 2, 3, 4; except 12, 13, 14)
• Many (Genitive Plural): 5 miesięcy, 12 miesięcy

Using a simple string for the months or years field forced users to choose one incorrect form for durations like "5 months" or "22 years".

Key Changes

1. Updated EnglishLocale to support both str (for backward compatibility) and dict[str, str] for complex mappings.
2. Created plural_rules.py to house the mathematical logic for selecting categories like one, few, and many.
3. Refactored compute_time_span_string in date.py to use a universal helper function, _get_localized_label. This ensures that years and months follow the exact same linguistic rules.
4. Added polish.yaml with grammatically accurate forms for all time-span scenarios.

How to add new complex language

After following instructions to add a new language new contributors should:

Identify plural rules

Check the https://www.unicode.org/cldr/charts/48/supplemental/language_plural_rules.html for target language (link is also in plural_rules.py).

Implement logic in plural_rules.py

Add language's numeric logic to plural_rules.py:

• Define a rule function: This function should take an integer n and return a string matching a CLDR category.
• Register the rule: Add your function to the PLURAL_RULES dictionary using its ISO 639-1 code as the key.

# e.g polish
def polish_rule(count: int) -> str:
    if count == 1:
        return "one"
    if 2 <= count % 10 <= 4 and not (12 <= count % 100 <= 14):
        return "few"
    return "many"


# Registry mapping ISO codes to rule functions
PLURAL_RULES = {
    "pl": polish_rule,
    # add here new set of rules
}

• Provide dictionary based translations
  In locale YAML (e.g polish.yaml) use dictionary for months and years fields (keys have to match strings returned rule function!)

# e.g polish
  months: 
    one: "miesiąc"
    few: "miesiące"
    many: "miesięcy"
  year: "rok"
  years:
    one: "rok"
    few: "lata"
    many: "lat"

Done!

Others

There is a default_rule as a fallback even if a specific rule of key is missing.

There is backward compatibility for simple string-based locales (English, German, etc.).