Reproduction steps:
1. Open Control Panel/ Region
2. In the Format dropdown menu, select German (Germany)
3. Select Additional Settings and set comma as decimal symbol
4. Apply the changes and open attached "case_1297805.zip" project
5. Select Window/UI Toolkit/UI Builder
6. In the Library window, select Project and expand Custom Controls (C#)/CustomDrawnVE_double
7. In the UI Builder Inspector, set Thickness to 1.1
8. Observe the value in UXML Preview

Expected result: Decimal symbol is a dot
Actual result: Decimal symbol is a comma

Reproducible with: 1.0.0-preview.1 - 1.0.0-preview.11(2019.4.16f1, 2020.1.17f1, 2020.2.0f1), built-in(2021.1.0a10)
Not reproducible with: 2018.4.29f1(UI Builder is not available)

Notes:
1. Issue is reproducible with German, Lithuanian, French, Spanish region formats
2. Issue is not reproducible with English, Chinese region formats