Is the Nintendo Switch region locked

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.”

-Source

Software-modding a Wii involves getting a copy of a game that contains a bug that can be exploited to run custom code. The first known hack involved a bug in Twilight Princess, for example, but the exploit doesn't work on Wiis with more recent firmware and other bugs had to be found. (I am not up on the current exploits, and being more specific would just make this answer out-of-date before long anyway.)

Bricking your Wii is unrecoverable without unusual technical skills. Bricking is also a risk you run any time you engage in soft-modding, so if you do decide to mod your Wii, do it only if you're willing to accept that you might destroy it.

Modding involves replacing parts of the operating system and firmware, so you can't update the firmware without losing the modding (and possibly having the update brick the machine in the process). However, the firmware has nothing to do with DLC like new songs and modding won't interfere with DLC. It's much more likely that a PAL game trying to access DLC servers in an NTSC region just won't connect properly, regardless of modding.