They must have change the trait of their helpers between 4.0 and 3.0. Here is the error :
help: the trait `for<'r, 'reg, 'rc, 's, 't0> Fn<(&'r Helper<'reg, 'rc>, &'reg Handlebars<'reg>, &'rc handlebars::Context, &'s mut RenderContext<'reg, 'rc>, &'t0 mut (dyn handlebars::Output + 't0))>` is not implemented for `FluentLoader<&StaticLoader>`
= note: required because of the requirements on the impl of `HelperDef` for `FluentLoader<&StaticLoader>`
They must have change the trait of their helpers between 4.0 and 3.0. Here is the error :