I'm French, so this will definitely color my judgement.
I find English to be rather simple. The grammar is as simple as can get, and due to it being the international language people are very open to weird accent (including ze frrrrench wane ;). This makes it easy to start, even with a pretty rough command of the language. Then the main challenge (IMHO) to a good mastery is the large vocabulary, and the very irregular pronunciation. But nobody expects that from a foreigner starting in English, which is a benefit of it being the international tongue I guess (spoken by all, mastered by... less).
On the other hand the French grammar is very complex. That's not specific to French, many languages are like this. Spanish and Italian are pretty close, and German is also complex (I tried the two first, but have no experience in the later). Still, it requires a significant upfront investment to get to a working level.
There is also a cultural challenge: the expectation in French is that you have to speak properly. For a foreigner there is some tolerance of course, but still the culture has a strong bias on speaking correctly, and for some professional contexts it will matter. There's a lot of history there, but let's say that command of language has long been a class differentiator. Nothing unique to French, I would guess it's the same in British English for Great Britons. But in an "international English" context that's definitely not the case, and it helps get started.
Another challenge is that words tend to be more overloaded in French (smaller vocabulary vs. English), which opens the door for a lot of double meaning. This exists in all language, but can be a challenge to a newcomer and is not as common in "international English".
So all in all, I'd guess international English is easier than French. In any case seriously learning another language and its culture is highly recommended, whatever your choice. It's really a mind opening experience, as any one will find that "obvious" things that everybody knows are actually just cultural conventions. So the more remote the culture, the bigger the gain on this account.
As for French dying, the number of speakers is actually increasing, mainly due Africa rising population and the large number of French speakers there.
This is perhaps one of the more insightful comments on learning a foreign language I've ever read. It really crystallizes the tolerance of the listener to foreign speakers as part of the barrier to learning a language.
One thing I'd add, at least as a native English speaker in a very international area, not all foreign accents and mistakes are equally well treated. And this may help contribute to and explain why some populations tend to learn English more poorly than others.
My wife is a non-native speaker (from Korea) and despite speaking and understanding English at relatively high levels, is clearly not a native speaker. But she frequently laments how people we know from different countries seemed to learn English faster or slower than others. It drives her a little crazy to think that all our Persian and Russian speaking friends effectively spoke fluent English inside of a few years (even if they have some lingering accentalisms like w-v confusion), while almost none of our Korean friends have managed the same level, even after decades.
But as an English listener, we definitely tolerate Persian and Russian accents better than Korean accents.
I find English to be rather simple. The grammar is as simple as can get, and due to it being the international language people are very open to weird accent (including ze frrrrench wane ;). This makes it easy to start, even with a pretty rough command of the language. Then the main challenge (IMHO) to a good mastery is the large vocabulary, and the very irregular pronunciation. But nobody expects that from a foreigner starting in English, which is a benefit of it being the international tongue I guess (spoken by all, mastered by... less).
On the other hand the French grammar is very complex. That's not specific to French, many languages are like this. Spanish and Italian are pretty close, and German is also complex (I tried the two first, but have no experience in the later). Still, it requires a significant upfront investment to get to a working level. There is also a cultural challenge: the expectation in French is that you have to speak properly. For a foreigner there is some tolerance of course, but still the culture has a strong bias on speaking correctly, and for some professional contexts it will matter. There's a lot of history there, but let's say that command of language has long been a class differentiator. Nothing unique to French, I would guess it's the same in British English for Great Britons. But in an "international English" context that's definitely not the case, and it helps get started. Another challenge is that words tend to be more overloaded in French (smaller vocabulary vs. English), which opens the door for a lot of double meaning. This exists in all language, but can be a challenge to a newcomer and is not as common in "international English".
So all in all, I'd guess international English is easier than French. In any case seriously learning another language and its culture is highly recommended, whatever your choice. It's really a mind opening experience, as any one will find that "obvious" things that everybody knows are actually just cultural conventions. So the more remote the culture, the bigger the gain on this account.
As for French dying, the number of speakers is actually increasing, mainly due Africa rising population and the large number of French speakers there.