Comment 3 for bug 158004

This has pretty much been fixed by implementing multi-architecture builders (bug 285206): builders now advertise support for all the architectures they're capable of building, and a large run of architecture-independent builds will therefore be distributed to all the i386/amd64/etc.-capable builders.

This isn't necessarily exactly what you asked for, as in theory there might be something like native ARM builders sitting idle which could also contribute. However, that's more subtle as allegedly "Architecture: all" builds often have assumptions of particular build architectures, usually but not always x86-flavoured. Bug 217427 tracks the more detailed work there.

In practice, though, what we have now is good enough for the scenario you mentioned. We have enough i386-capable builders now that even a language pack run is dealt with fairly promptly.