Nous nous sommes fait expliquer un autre itinéraire par la réceptionniste, car le GPS proposait un détour trop long.

They do two different jobs:

• The first nous is the subject: we.
• The second nous is part of the pronominal structure se faire + infinitive, which often means to get something done for oneself / to have something done for oneself.

So Nous nous sommes fait expliquer... means something like:

• We had ... explained to us
• We got ... explained to us

That second nous is not redundant; it shows that the action was done for us / to us.

This is a very common French pattern.

se faire + infinitive often means:

• to get something done
• to have something done
• sometimes to let oneself be ... depending on context

In this sentence:

• nous nous sommes fait expliquer un autre itinéraire = we got another route explained to us

Other examples:

• Je me suis fait couper les cheveux.
  = I got my hair cut.

• Elle s’est fait aider par un ami.
  = She got helped by a friend.

So here, the idea is not that we explained something, but that someone explained it to us.

Because after faire in this causative structure, the second verb stays in the infinitive.

So French uses:

• faire expliquer
• faire réparer
• faire venir
• faire traduire

Not:

• faire expliqué
• faire réparé
• faire venu
• faire traduit

That is why the sentence has:

• nous nous sommes fait expliquer

not

• nous nous sommes fait expliqué

Because in the construction faire + infinitive, the past participle fait is normally invariable.

So even with nous, French still writes:

• Nous nous sommes fait expliquer...

not

• Nous nous sommes faits expliquer...

This is one of the classic things learners notice, because with many other verbs in the passé composé, agreement might appear. But with faire + infinitive, standard French keeps fait unchanged.

It is the thing being explained.

So the underlying idea is:

• La réceptionniste explique un autre itinéraire à nous.
• The receptionist explains another route to us.

In the actual sentence, this becomes the causative structure:

• Nous nous sommes fait expliquer un autre itinéraire par la réceptionniste.

So:

• un autre itinéraire = the route that was explained
• nous = the people receiving the explanation
• la réceptionniste = the person doing the explaining

Because la réceptionniste is the person who performed the action of explaining.

Here, par works like by in English:

• explained by the receptionist

That helps distinguish roles:

• nous = the people who received the explanation
• par la réceptionniste = the person who gave the explanation

If you used à la réceptionniste, that would normally suggest the receptionist is the one receiving something, which is not the meaning here.

Proposait is the imperfect of proposer. It is used here to give background or describe what the GPS was suggesting at the time.

So the sentence is structured like this:

• We got another route explained to us,
• because the GPS was suggesting a detour that was too long.

The imperfect works well because it presents the GPS’s suggestion as part of the situation or background reason.

If you used a proposé, that would sound more like a single completed event:

• the GPS suggested

That is possible in some contexts, but proposait is very natural here because it describes the ongoing situation that led us to ask for another route.

French uses articles much more often than English does.

So where English might say:

• GPS was suggesting...

French usually says:

• le GPS proposait...

That definite article is normal and idiomatic in French. It refers to the GPS device or navigation system involved in the situation.

Because long here is an adjective describing the noun détour.

• un détour trop long = a detour that is too long

Longtemps is an adverb meaning for a long time, so it would not fit here.

Compare:

• un trajet long = a long trip
• ça dure longtemps = it lasts a long time

In this sentence, the detour itself is being described as excessive in length, so trop long is correct.

Both can mean because, but they are not always identical in tone.

• car is often a bit more formal, literary, or written
• parce que is more common in everyday speech

So:

• ..., car le GPS proposait un détour trop long.
• ..., parce que le GPS proposait un détour trop long.

Both are understandable here. Using car gives the sentence a slightly more polished or written feel.

Yes. A very natural alternative would be:

• Nous avons demandé à la réceptionniste de nous expliquer un autre itinéraire...

That means:

• We asked the receptionist to explain another route to us...

The original sentence with nous nous sommes fait expliquer is also natural, but it focuses more on the result: we got the explanation.

So the difference is roughly:

• Nous avons demandé... = focuses on the request
• Nous nous sommes fait expliquer... = focuses on obtaining the explanation

Both are useful, but the original sentence is a great example of a very common French structure.