This is probably a dumb question, but I’m going to ask anyways. Does knowing a foreign language carry a decent amount of weight when applying for an airline?

In the US, it carries essentially zero weight. Might be nice resume color, or interview topic of interest though.

English the language used world-wide for aviation, so that's the language you need as a pilot.

Flight attendants can get hired for language skills, and then may be selectively assigned to certain routes (I have a young friend who speaks french, she was hired for that and flies almost exclusively to France, the other FA's all very senior).

Oh ok. But if it helps my resume then I guess it will count for something. I’m studying French.

English is widely used in aviation only because the allied forces won WW2.
Other ICAO approved languages are Russian, Chinese, Spanish and French.

So when flying in South America you will hear Spanish a lot on the radio.
Only airlines from non-Spanish speaking countries will use aviation English.
In China you’ll hear 80% Chinese unless you’re a non-Chinese airline and you’ll use aviation English.

So you’ll never need your language skills. Only value on your resume would be ‘general’ interest.

It'll get you a throw away question on your interview, and that is about it. I speak Korean due to a couple years I spent there and it warranted a quick question during app review at my last interview. Good conversation starter at least.

Why?
Other than Latin, is there a less useful language on the planet in 2018?

Yes. Neomelenesian. Ask how I know.

French is a useful language, and for many years was the international diplomatic language. It's still widely spoken and is quite useful in Europe.

While airlines don't value a second language in a pilot, it's never wrong to invest time and effort in learning another language. It's not uncommon for people in many foreign areas, particularly Europe, to speak five or more languages.

Nationalistic ignorance is typical of many in the US who couldn't be bothered.

Take the time. Learn another language.

French is an official language in 29 other countries. I don’t know if you’ve traveled much outside the US, but it’s quite common to hear French spoken in airports

I think it could be beneficial for an airline on the customer service aspect. I read a PSA Airlines pilot profile the other day of a CRJ Captain, she said she regularly helped French speaking passengers in the airport and aboard the aircraft before and after the flight. I think maybe it could carry some bonus points.

So do you know neomelenesian? I thought that was a form of cancer or something. And yes, it sounds less useful than french.