Open up a Terminal window. (If you have never seen Terminal before, go to Spotlight Search and type "Terminal.")
In the Terminal box, try this:
defaults read com.google.Chrome AppleLanguages
If you see this:
The domain/default pair of (com.google.Chrome, AppleLanguages) does not exist
... don't panic. It just means that nobody else has used this method before. To change your language, do this:
defaults write com.google.Chrome AppleLanguages '(pt-BR)'
Capitalization of the language code you enter is unimportant; your system will normalize it to lower-lower-UPPER-UPPER.
Here I've chosen Portuguese as it is spoken in Brazil. Feel free to enter the IETF language code of your choice; some of my personal favorites are en-UK, fr-CA, and es-MX. For a complete list, see the Wikipedia entry.
Restart Chrome.
If Chrome knows the language you entered, the top menu will immediately switch; mine says Arquivo, Editar, and Visualizar when I switch to pt-BR.
To switch back, run the same Terminal command with your preferred language. And if you're ever confused about what language has been selected, do this:
defaults read com.google.Chrome AppleLanguages
... or use a Web inspector to enter console mode on any page and type this:
window.navigator.language
And for Chromium use
org.chromium.Chromium