Are there any appearance changes to 3DS cartridges for the Nintendo Selects incarnations

The 3DS, unlike the Nintendo DS before it, is region-locked. There are 3 regions: North America, Japan and EU/Australia. So if you buy your 3DS in Australia, you'll need to purchase games from either the EU or Australia as well. Nintendo explained their reasoning in the following statement:

“Nintendo 3DS hardware is available in three versions: Japanese, American and European/Australian,” Nintendo told us in a statement.

“Nintendo has developed different versions of Nintendo 3DS hardware to take into account different languages, age rating requirements and parental control functionality as well as to ensure compliance with local laws in each region. Nintendo 3DS also offers network services specifically tailored for each region.”

It went on: “Additionally we want to ensure the best possible gaming experience for our users and there is the possibility that Nintendo 3DS software sold in one region will not function properly when running on Nintendo 3DS hardware sold in another.”

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As far as content is concerned, no. You're getting the same game that the previous buyers got, it's just a different box.

While there may sometimes be bugfixes, there does not have to be. Any bugfixes introduced in the Nintendo Selects version are necessarily minor, and would not affect normal gameplay. This is necessarily the case because there is no patching mechanism for the already released copies; if there are major bugs which severly impact the gameplay (such as the freezing bug in PAL copies of Super Paper Mario), the only way to fix them is by replacing the disc or cartridge.

Additionally, any bug fixes are not made specifically for the Nintendo Selects version, and may have already been released in previous pressings. Whether or not the two coincide is a question of when they switched the master copy; that may happen at the same time as pressing additional copies for the Nintendo Selects version, or it may not.

TL;DR: There are no notable differences.