New Install with php 3.1 RC4 (English) and added the NL (latest) language package.

The Custom Field (Only Dropdown) has Error Issues

Please see also the discussion at:
https://www.phpbb.com/community/viewtopic.php?f=466&t=2264111
With pictures and test results.

Made a test with adding new test Custom Fields:

Results with English + Dutch language

And User (preference) = English Language (is the below Result)
And User (preference) = Dutch Language (is the below Result)

Booleans (Yes/No) - OK
Date - OK
Dopdown - Box - ERROR !!!
Google+ - OK
Numbers -OK
Single Text Field - OK
Textarea - OK
URL (link) - OK

Important: It only happens when you Edit a Field (after it has made and saved)

Step 1:
So, try to make a custom field, add the different languages in the [EN] and [NL] Option fields, and then Save it.
Step 2:
Try to change the fields at the [EN] and [NL] and SAVE it.
You will see, the fields have not been changed and also the Error again.