If you save the following file, numbers.lng to the Opera directory, and choose "English [Build-904] Numbers" as the user interface language in Opera preferences, then all strings will appear preceded by their string number.
This is very useful when translating, because you can often tell better what translation is appropriate when you see the string in context. I find that the ideal is to have two computers side by side, one with Opera showing the string numbers, and another one where I am working on and testing the translation.
The technique is a bit crude. Sometimes the space available isn't enough to show the whole string number. Sometimes the string number messes up the formatting a bit. But it works for the majority of cases.
Something to be careful of is that there are a few strings numbers (about 25) which appear again for a different string in a different section of the language file. (e.g. string number 4001 appears in both the [Translation] and [Menu] sections). So you have to make sure you put the right translation in the right place.
I used a Word macro to copy the strings numbers into the strings, although there are no doubt better methods.
Caoimhín Ó Donnaíle,
Sabhal Mór Ostaig