Steps to reproduce:
1. In Ubuntu, go to System settings -> Language support -> Regional formats
2. Select a language which uses commas (,) instead of dots (.) for decimal separators (ie Lithuanian, German)
3. Apply it system-wide and re-log to the user account
4. Open any Unity project
5. Select any GameObject in the Hierarchy window
6. Set X value of transform to 0.5

Expected result: value is displayed as 0.5
Actual result: value is displayed as 0,5

Reproduced on: 5.5.3f1, 5.6.3f1, 2017.1.0f3, 2017.2.0b6

Note: this issue seems to be similar to Case 930798, however it occurs on .NET 3.5 as well. No repro on macOS.