Reproduction steps:
1. Open the attached “IN-104953.zip” project
2. Open the “SampleScene”
3. Select “English (en).asset” from the Project Window
4. In the Inspector, enable the “Use Custom Formatter” property
5. From the Hierarchy, select the ‘Canvas > Example’ GameObject
6. In the Inspector, find the “Localize String Event” Component
7. Find the “Local Variables” field
8. Change either or both “Integer” values to 5 (or higher)
9. Disable and re-enable the “Example” GameObject to trigger a refresh of the String
10. Observe the Game view

Expected result: Text is correctly pluralized following the pluralization rules of the current locale
Actual result: Text is rendered as such “The brown fox{foxcount:p:|es} jumps over the lazy dog{dogcount:p:|s}”, following the locale’s custom formatter override

Reproducible with: 1.4.5, 1.5.4 (2022.3.63f1, 6000.0.51f1, 6000.1.8f1, 6000.2.0b5)

Reproducible on: Windows 11
Not reproducible on: no other environment tested

Notes:
- Also reported in discussions: https://discussions.unity.com/t/pluralization-rules-custom-formatters-issue/1654890
- In the example, English is the current locale and text is displayed with "The brown foxes jumps over the lazy dogs"
The amount of "foxes" and "dogs" can be changed in the Inspector of the “Examples” text GameObject. Changing the values will correctly pluralize the amount of foxes and dogs in the sentence
- The issue most likely stems from the "PluralLocalizationFormatter" script, which handles pluralization based on the locale. Here it is incorrectly enforcing Russian pluralization rules, because the formatter override is set to "ru-RU", despite the current locale actually being English. This is because Russian pluralization has three different forms: one, few, and many. In the string, no third option (aka "many") is defined, so it fails (5 being considered "many")