The CMD shell decodes batch scripts using the attached console's output codepage, which defaults to OEM. OTOH, venv writes the replacement values for the template activate.bat as UTF-8 (codepage 65001), which is correct and should not be downgraded to OEM. 

Instead, the batch script could temporarily switch the console to codepage 65001. Then restore the previous codepage at the end. For example:

    @echo off
    for /f "tokens=2 delims=:" %%a in ('"%SystemRoot%\System32\chcp.com"') do (
        set "CODEPAGE=%%a"
    )
    "%SystemRoot%\System32\chcp.com" 65001 > nul

[rest of script]

    "%SystemRoot%\System32\chcp.com" %CODEPAGE% > nul
    set "CODEPAGE="
    :END