À la boutique, la vendeuse demande ma pointure et cherche une paire avec une semelle plus souple.

Yes. À la boutique means at the shop or in the shop in this context.

French often uses à with places in a broader, more general way than English does. So:

• À la boutique = at/in the shop
• Au magasin = at/in the store

Starting the sentence with À la boutique sets the scene first: At the shop, ...

The comma shows that this opening phrase is just background information.

Both can refer to a shop, but they are not always identical in tone.

• boutique often suggests a smaller shop, sometimes a more specialized or stylish one
• magasin is a more general word for store/shop

So boutique can feel a bit more like shop or boutique store, while magasin is broader.

Because vendeuse is the feminine form of vendeur.

• un vendeur = a male sales assistant / salesman
• une vendeuse = a female sales assistant / saleswoman

Since the sentence is talking about a woman, French uses la vendeuse.

Demander means to ask for or to ask.

So:

• la vendeuse demande ma pointure = the sales assistant asks for my shoe size

In French, demander can take a direct object:

• demander mon nom = ask my name
• demander mon âge = ask my age
• demander ma pointure = ask my shoe size

You do not need à moi here. That would sound unnatural.

Pointure means shoe size.

It is the normal French word used when talking about the size of shoes:

• Quelle est votre pointure ? = What is your shoe size?
• Je fais du 39. = I’m a size 39.

It is not usually used for clothing sizes in general; it is specifically for shoes.

Because pointure is a feminine noun:

• une pointure

So the possessive adjective must also be feminine:

• ma pointure = my shoe size

Compare:

• mon sac because sac is masculine
• ma veste because veste is feminine

Une paire means a pair, and in this context it means a pair of shoes.

French often uses une paire when talking about shoes because shoes naturally come as a pair.

So:

• chercher une paire = look for a pair
• chercher une paire de chaussures = look for a pair of shoes

Here, de chaussures is omitted because it is understood from the context.

It can mean either, depending on context.

French uses the simple present tense more broadly than English does. So:

• elle cherche can mean she looks for
• or she is looking for

In this sentence, because it describes what is happening in the shop right now, English would most naturally say is looking for.

Une semelle is a sole, as in the bottom part of a shoe.

Souple means flexible, soft, or supple, depending on context.

So:

• une semelle plus souple = a softer / more flexible sole

In a shoe-shopping context, more flexible sole is often the most natural translation.

Plus is used to make a comparison: more.

So:

• souple = flexible / soft
• plus souple = more flexible / softer

This is the regular French pattern for comparatives:

• plus grand = bigger / taller
• plus petit = smaller
• plus cher = more expensive

The sentence is saying the sales assistant is looking for a pair with a sole that is more flexible than another one already considered or than usual.

Yes, but French often uses the singular when talking about the characteristic design of a shoe or pair of shoes.

So une paire avec une semelle plus souple means the pair has a softer sole type/design. It is not focusing on counting both soles separately.

English can do something similar:

• a shoe with a rubber sole even though a pair of shoes has two soles.

It does agree with semelle, but for this adjective the feminine form looks the same as the masculine form.

• masculine: souple
• feminine: souple

So the agreement is there grammatically, but there is no visible spelling change.

For comparison:

• petit → petite
• grand → grande
• souple → souple

Yes, very normal.

French often begins a sentence with a place or time expression to set the scene:

• À la boutique, ...
• Au restaurant, ...
• Le matin, ...
• En été, ...

After that, the main clause follows:

• la vendeuse demande ma pointure...

This is a natural and common sentence structure in both written and spoken French.