Des de fa dos mesos, la meva germana treballa a una oficina de la ciutat.

It means for two months now or since two months ago.

This Catalan pattern is very common for an action that started in the past and is still continuing now:

• des de fa dos mesos = for the last two months
• des de fa una setmana = for a week now
• des de fa anys = for years now

A useful way to remember it is:

• fa + time = ago
• des de fa + time = for / since ... ago until now

So here, the idea is that the sister started working there two months ago and still works there now.

Because the action began in the past but continues in the present.

In English, we usually say:

• My sister has been working... for two months

In Catalan, it is very normal to use the present tense in this situation:

• La meva germana treballa... des de fa dos mesos

So Catalan often uses the present where English prefers the present perfect continuous.

In Catalan, possessives are very often used together with the definite article:

• la meva germana = my sister
• el meu pare = my father
• la nostra casa = our house

This is one of the things that feels unusual to English speakers, because English normally does not use the before my, your, etc.

So la meva germana is the normal Catalan pattern.

Because germana is a feminine singular noun.

The possessive must agree with the noun:

• el meu germà = my brother
• la meva germana = my sister

So:

• meu = masculine singular
• meva = feminine singular

Here a means in / at.

With treballar, Catalan often uses a before a place:

• treballar a una oficina = to work in an office
• treballar a Barcelona = to work in Barcelona
• treballar a l’escola = to work at the school

So although English often says in an office, Catalan uses a.

Because they do different jobs:

• a = the preposition, meaning in/at
• una = the indefinite article, meaning a/an

So:

• a una oficina = in an office

This is completely normal in Catalan, just like:

• a una casa = in a house
• a una escola = at a school

Yes. Oficina is a feminine noun in Catalan, so it takes:

• una oficina
• l’oficina
• aquesta oficina

Many nouns ending in -a are feminine, and oficina is one of them.

It means of the city or more naturally in English, in the city / from the city, depending on context.

In this sentence, una oficina de la ciutat most naturally means an office in the city or a city office.

So the phrase describes what kind of office it is.

A few similar examples:

• un museu de la ciutat = a museum in the city / the city’s museum
• un barri de la ciutat = a neighborhood of the city

Yes. Catalan word order is flexible.

For example, these are also natural:

• La meva germana treballa a una oficina de la ciutat des de fa dos mesos.
• Des de fa dos mesos treballa a una oficina de la ciutat.

Putting Des de fa dos mesos at the beginning gives that time expression a bit more emphasis.

The version you were given is very natural and common.

Because that opening time phrase is being set off from the rest of the sentence.

Catalan often uses a comma after an introductory element such as:

• a time phrase
• a location phrase
• a linking expression

So the comma here is natural and helps readability:

• Des de fa dos mesos, la meva germana...

You may also sometimes see similar sentences without a comma, especially in less careful writing, but the comma is a good standard choice here.

Not in this sentence if you want the same meaning.

• des de fa dos mesos means the action has continued for two months up to now
• fa dos mesos by itself means two months ago

Compare:

• Treballa aquí des de fa dos mesos. = She has been working here for two months.
• Va començar a treballar aquí fa dos mesos. = She started working here two months ago.

So des de fa is the right choice when the action is still continuing.

A rough pronunciation guide for an English speaker could be:

dez de fa doz muh-ZOOS, luh MAY-vuh jer-MAH-nuh truh-BAH-yuh ah OO-nuh uh-fee-SEE-nuh de luh see-oo-TAT

A few helpful notes:

• germana has a soft g sound, like the j in genre
• ll in treballa is often pronounced like a y sound in many varieties
• ciutat is pronounced in three syllables: ciu-tat

Exact pronunciation varies a bit by dialect, but that rough guide will get you close.