What version are you running?
2.5.1.1
What's the URL of the page containing the problem?
Multiple (see screenshots)
What steps will reproduce the problem?
Open reviewboard in Chrome (where both the OS and browser default language are set to English, but localized to the Netherlands).
What is the expected output? What do you see instead?
I expected all the labels and text to be in English.

However a few labels are in Dutch instead ('Alle' instead of 'All', 'Afmelden' instead of 'Log out' and 'Gebruikersnaam' instead of 'Username').
What operating system are you using? What browser?
Windows 10 Pro 64 bit

Chrome 48.0.2564.103 m

Chrome plugin: Quick Language Switcher 0.0.0.4
Please provide any additional information below.
I found a workaround for this problem by using a Chrome plugin: Quick Language Switcher.

If I set this to English all the labels are properly set to English.

If I use the same tool to set the language to Dutch I am back to the original situation and therefor I assume that not all translations to Dutch are available either.