Here's an example from the homepage:

Gebruik de ongebruikte tijd van je computer (Windows, Mac, Linux of Android) om ziektes te genezen, de opwarming van de aarde te onderzoeken en voor andere wetenschappelijk projecten. Het is veilig en makkelijk:

Kies projecten
Download BOINC software
Vul een e-mail adres en wachtwoord in.

O, si usted ejecuta varios proyectos, pruebe un administrador de cuentas tales como GridRepublic o BAM!.

For Android devices, download the BOINC or HTC Power To Give app from the Google Play Store.

I'm seeing English, Spanish, and Dutch intermixed throughout the site. My system language is set to English, although I have the others available as keyboard maps and secondary display languages (since I can speak them).

I suspect this has something to do with a conflict between your website's attempt to guess my language and my desire for location-anonymity.

I'm in The Netherlands, have set my Windows time and date to UTC(+01:00) Amsterdam... time. Windows is set to English.

Region and Language Format is set to Dutch.
Date and time follows European settings.
Location: Netherlands.
Keyboard and Languages->General->Default input language English (United States).
Keyboard and Languages->General->Installed services: English, keyboard US.
Keyboard and Languages->Display language: English.

Boinc.berkeley.edu language is set to English.
Browser (Pale Moon) character encoding set to Unicode (UTF-8).

That text:

Use the idle time on your computer (Windows, Mac, Linux, or Android) to cure diseases, study global warming, discover pulsars, and do many other types of scientific research. It's safe, secure, and easy:

Choose projects
Download BOINC software
Enter an email address and password.

Or, if you run several projects, try an account manager such as GridRepublic or BAM!.

For Android devices, download the BOINC or HTC Power To Give app from the Google Play Store.

But, I do see that that last sentence is not translated when checking the site with Internet Explorer 11. It shows in English in an otherwise completely Dutch page. So we can easily deduct that that sentence hasn't been translated yet and therefore shows in the original language it is in.

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