There are various single-phase power socket standards in use. Some countries use plugs that can be plugged in the socket only in a specific orientation (for example, the United Kingdom) while other sockets accept the plug in either orientation (for example, Germany).
British plug:
German plug:
Do engineers have to account for this fact when designing electrical devices? Could certain designs be simplified if line and neutral always were clearly defined?
No, there really isn't any significant simplification possible. When you interface to the power line, you have to make your design as robust and fail-safe as possible, in order to avoid product liability issues.
Even if the design of the wall socket permits only one physical orientation, there are enough mis-wired wall sockets out there that you really need to treat both Line and Neutral as "hot" inside your product.
In all of the mains connected projects/products I've worked with, I specify a 3 prong connector where possible. This only works in one orientation and provides a dedicated ground (unless circumvented by the building wiring or an adapter I'll link to below)
Line and neutral should always be defined, but for some applications it does not matter. Think small holiday lights or some power tools.
When consumers are involved, it can come down to case grounding. If the product is designed inside a conductive metal case, the case can be left floating or ideally grounded. Without knowing which conductor is neutral or ground, the case could potentially be at the hot potential.
