The bug

It appears the new language files are not read using UTF-8 (without BOM) as encoding or the game actively encodes the read content using UTF-8 again.

For example € (\u20AC) is displayed as € which is its UTF-8 encoded representation.

How to reproduce

1. Download the attached file MC-124179 resource pack.zip and place it in your resourcepacks folder
2. Load the resource pack
3. Use the following command

   /tellraw @s {"translate":"utf_8_test"}
   

The language file is properly encoded using UTF-8 without BOM and contains the following:

{
  "utf_8_test": "✓ (check mark)\n★ (star)\n☂ (umbrella)\n€ (Euro sign)"
}

But in-game the following is shown:

✓ (check mark)
★ (star)
☂ (umbrella)
€ (Euro sign)