Nirvana's release artwork was designed in the USA. This included not only the front cover, but also the back cover with all the legal text, catalog number, barcode, and more. The original information was only valid in the USA, so in other countries they had to edit those parts. [1,2]
A printing film and a chromalin were sent to other countries. A chromalin is a high quality card color print of the artwork laid out flat without any folds or cuts.  Chromalins for Nevermind and Smells Like Teen Spirit can be seen in images 1-6, and some more can be found on the Pennyroyal Tea chromalins page. Note that the Smells Like Teen Spirit chromalin was made only for the UK . No digipak was released in the USA, but it was still designed there. All the chromalins shown here were sent to Geffen UK.
The chromalin received from USA was put under a transparent sheet, and the record label would write handwritten amendments on the transparent sheet, correcting catalog numbers, barcode numbers, and other details for use in their country, as shown on the Smells Like Teen Spirit chromalin below. The film and chromalin were then sent to the art department, where the chromalin was used as a guide to perform proper amendments to the printing film by erasing the American details and adding, in this case, the British details. Today this is performed on layered computer images. 
The amended film was then sent to be printed, along with a new chromalin printed by the art department. The new chromalin served as a color proof for the printing company which would print the inserts or sleeves. The American chromalin was kept by the Geffen office or discarded. 
To remove the original text, they also had to remove and recreate the background before entering the new text. This was not always seamless. Image 7 shows the German Nevermind (GEF 24425) sleeve. "Printed in Germany" has been printed in an area where there originally was no text on the American release, and is therefore seamless. The Dutch (GEF 24425) and Croatian (LP-7-1 2036318) sleeves shown in images 8-9 appear to be edited versions of the German sleeve, with little effort to recreate the original background after "Printed in Germany" was removed.
Often some text was not amended at all. For example, the French Lithium 7" single (GES 19134) has "Made in USA" written on the back, even though it was entirely manufactured in France. The labels have all the proper French credits, as the labels were designed from scratch in France. MCA credits are also frequently not removed from reissues, and appear along with Universal credits. In some cases, the new Universal credits on reissues from 2000 and on are printed in better quality than the old text, in Europe at least. It seems they lost the original films and had to clone their own artwork.
1. American Nevermind chromalin
2. American Nevermind chromalin, front part
3. American Nevermind chromalin, back part
4. American Smells Like Teen Spirit chromalin
5. American Smells Like Teen Spirit chromalin, front part
6. American Smells Like Teen Spirit chromalin, back part
7. German Nevermind sleeve detail
8. Dutch Nevermind sleeve detail, edited version of the German sleeve
9. Croatian Nevermind sleeve detail, edited version of the German sleeve