That's not a meaningful distinction. Julia is a powerful enough language that OffsetArrays don't need to be built in to work perfectly. Good languages don't need everything built in.
The comment above states: "...Julia that have arbitrary array indexing as a core feature...". Is an external package written in Julia considered a "core" feature of the Julia language? Why does the Julia community always twist the facts? Why cannot they accept the facts as they are? It would not reduce the value of their efforts, I guarantee you—decency and honesty always prevail.
The base standard library and "idiomatic" Julia are both largely written without the assumption of 1-based indexing, instead dispatching to what you think about as an array-oriented dsl, which is this interface: https://docs.julialang.org/en/v1/manual/interfaces/#man-inte...
It is very normal in Julia to implement such interfaces to do what you want. Julia uses these types of internal interfaces with multimethods in lieu of a lot of compiler intrinsics.
So I would say Julia and Fortran have different implementations of arbitrary indexing.
They are both oriented around core design features of each respective language.
External Julia packages can impliment core features. For another example, look at StaticArrays. Fortran is a brittle old language where everything supported must be built into the language. Julia isn't.
Again, twisting facts by some Julia novice about a topic they have no familiarity with. Fortran does not require features to be built-in. Look at all the intrinsic Fortran modules. But array-based indexing IS built-in to the language. Unlike Julia, you do not have to rely on an external package for it. Perhaps the Julia community's relatively young age has made it so arrogant, Machiavellian, and fact-twisting about other languages to prove their presence here and there. Other programming communities are old enough to put decency and honesty in their core values.
Julia's compiler is made to be extendable. GPUCompiler.jl which adds the .ptx compilation output for example is a package (https://github.com/JuliaGPU/GPUCompiler.jl). The package manager of Julia itself... is an external package (https://github.com/JuliaLang/Pkg.jl). The built in SuiteSparse usage? That's a package too (https://github.com/JuliaLang/SuiteSparse.jl). It's fairly arbitrary what is "external" and "internal" in a language that allows that kind of extendability. Literally the only thing that makes these packages a standard library is that they are built into and shipped with the standard system image. Do you want to make your own distribution of Julia that changes what the "internal" packages are? Here's a tutorial that shows how to add plotting to the system image (https://julialang.github.io/PackageCompiler.jl/dev/examples/...). You could setup a binary server for that and now the first time to plot is 0.4 seconds.
Julia's arrays system is built so that most arrays that are used are not the simple Base.Array. Instead Julia has an AbstractArray interface definition (https://docs.julialang.org/en/v1/manual/interfaces/#man-inte...) which the Base.Array conforms to, and many effectively standard library packages like StaticArrays.jl, OffsetArrays.jl, etc. conform to, and thus they can be used in any other Julia package, like the differential equation solvers, solving nonlinear systems, optimization libraries, etc. There is a higher chance that packages depend on these packages then that they do not. They are only not part of the Julia distribution because the core idea is to move everything possible out to packages. There's not only a plan to make SuiteSparse and sparse matrix support be a package in 2.0, but also ideas about making the rest of linear algebra and arrays themselves into packages where Julia just defines memory buffer intrinsic (with likely the Arrays.jl package still shipped with the default image). At that point, are arrays not built into the language? I can understand using such a narrow definition for systems like Fortran or C where the standard library is essentially a fixed concept, but that just does not make sense with Julia. It's inherently fuzzy.
Much of Julia language is written in Julia. So the 'external package' is just an extension you install/use if you wish. A distinction without a difference.