[BUG] I18n.fallbacks should not be mutable

[alexfiennes]

What I tried to do

teamskills(dev)> I18n.locale
=> :en
teamskills(dev)> fallbacks = I18n.fallbacks[I18n.locale]
=> [:en, :"en-US"]
teamskills(dev)> fallbacks.concat([:wtf])
=> [:en, :"en-US", :wtf]
teamskills(dev)> I18n.fallbacks[I18n.locale]
=> [:en, :"en-US", :wtf]

What I expected to happen

I would either expect that the array that is returned from I18n.fallbacks is a dup of the original one so that I can mutate it without poisoning future requests, or that the array that is returned is frozen so that it can't be mutated.

I personally would prefer the frozen array so that we don't incur the overhead of duplicating the array on each request if we probably aren't going to mutate it.

I can make a PR once we have decided which behaviour we prefer.

What actually happened

The array that is returned is mutable and if I mutate it then the mutated array is returned for future requests.

Versions of i18n, rails, and anything else you think is necessary

i18n version 1.14.7
rails 7.2.2.1
ruby 3.4.2

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Bonus points for providing an application or a small code example which reproduces the issue.

Thanks! ❤️