Not a language issue per say then. It shouldn't a problem to add support for this. Julia is even designed from the start to work with Strings of different types. Something not common in most other languages I've worked with.
Most string functions operate on AbstractString types. You can create whatever subtype you like implementing alternative string encodings.
Building on this comment, it's actually surprisingly straightforward to add your own encodings into the core language. Just add the appropriate types and extend the necessary base methods.
In this case, it appears Julia 0.5 has a transcode method supporting UTF8/16/32. [1]
I'd venture a guess that implementing any string encoding support in a language I'm not that familiar with would likely be quite an undertaking.
If not, then it also begs the question - if it's just that simple, why's it not already there in the core alongside UTF-8?
Anyone new to the language working through data problems is going to pick it up and start at the data collection/wrangling stage, and immediately run into the 'you can implement that yourself/run it through a converter' mentality, and likely go back to whatever they were using before. I know we did.
Most string functions operate on AbstractString types. You can create whatever subtype you like implementing alternative string encodings.