What's the best way to signal that I'd prefer to be served in English?

Traditionally, one would just point and shout their requests in English… As a nation, it’s served us well for at least 600 years.

The more advanced may wish to transpose a few words into the local language, numbers, ‘please’ and ‘thank you’ being the most common

For example in French:

UNE OF THOSE SIH VOOO PLAY

I think that “I don’t speak this language well” is a phrase worth learning. That way you at least engage the person in their native language but state your preference.

For your Korean example it would be:
“Han-gu-geo-rul chael mo-te-yo”

I think it’s actually important to specify the place instead of making the question generic, because it does matter. For example:

• Montreal is a locale where most people are bilingual with French and English. It’s perfectly acceptable to reply “hello” when greeted “bonjour”, and in fact, on the street you’re better off just opening your mouth and speaking English instead of asking “Do you speak English?” because the latter doesn’t get to the point (beggars ask “Do you speak English?” in order to engage you in conversation before you realize what they’re really after).

• In Japan you should follow the advice that others have given you and make an effort to at least use small talk words in Japanese. It shows your effort and that you are not inflexible. On the other hand, if you look like a foreigner, lots and lots of people will assume you don’t speak Japanese anyway, so they may greet you in English to begin with. More likely, they will avoid you because they don’t want to become responsible for a conversation they can’t carry on. Korea might be similar, I don’t know.

• If you are a tourist in Nepal I don’t think you need to worry too much about not speaking Nepalese: so few foreigners do. Just say “namaste” and you’ll be fine.

• In Europe (like France, Germany) you might annoy people if you expect service in English, as others have said. I don’t have much experience there, but my guess is that you should go out of your way to demonstrate an effort if possible.

• Other places, other customs.

It appears your situation is somewhat specific to a metropolitan area of South Korea. This country can be tricky in some cases, especially if your appearance is Asian but clearly not of Korean stock.

As someone who was once compelled to study several languages and then lost speaking fluency in many but somehow retained literacy (so I get sent to X, Y and Z frequently) I can feel your pain. My advice is to learn a few phrases that explain your situation in the most polite dialect available.

In particular:

• “I’m sorry, I do not speak ____. Do you speak English?”
• “Do you speak English?”
• “I’m sorry, I do not speak ____ well. More slowly?”

and whatever else might come to mind and seem reasonable. Most of the time if they initiate the conversation it winds up like:

Them: [Snoopy parents…]

You: [I’m sorry, I do not speak ____. Do you speak English?]

Them: “Ah, you’re not from here! Anyway, so you want cheese or onions on that?”

and life goes on. If you initiate, try to go as far as you can in the local language and you’ll accidentally get much better at it daily.

Anyway, there are worse things in life than being embarrassed or having your feelings hurt.

This is much the same as has already been written by Mark Mayo but I have heard some arguments in favour of duplication of answers (basically that one answer may reinforce the other).

I think inability to speak the local language is nothing to be embarrassed about. There are well over 100 “major” ones and nobody speaks all of these. Staying away until you have learnt the language of the country you propose to visit does nobody any favours.
If you speak English bear in mind that, being the “lingua franca”, you are very lucky there is a good chance that whoever you are trying to converse with will welcome the opportunity to practice their English. However that should not be presumed (especially in France!) and it is only courtesy to make some effort to speak the local language.

So I would recommend commencing a conversation in the local language where at all possible. It shows respect but also, from the accent and speed, may help someone who might be expecting you to be fluent in their language to realise early on that you are not. It is less of a shock to the person to whom you are talking, who might even volunteer to switch to English without further prompting because they appreciate you made the effort and so are more inclined to help you. Whereas a few people fluent in English as a second language may assume deafness if affronted by lack of consideration.

I would not chose to start with “a clear English reply of “Good morning””. If you are around for long enough the local language you kick off with might gradually be extended from say (in Korean or whatever) “Good Morning” to “Good Morning, did you sleep well” or “Good Morning, it is a fine day” or whatever is appropriate. That should gradually enhance your grasp of the local language and build confidence in it more effectively than never practicing even if learning from a book/audio or taking lessons. I was forced into an approach of this kind and after only a couple of months was very surprised to find myself being called on to provide translation services (of a kind).
Murdering the grammar and an atrocious accent is not nice but for most a much quicker route to passable fluency than meticulous attention to the detail of both. In musical terms, the discipline of scales may lead to concert pianist standard in due course but plonking away on a keyboard may achieve something pleasant enough to listen to sooner. Selling the piano however may mean no progress at all.

I repeat that lack of fluency outside your home country is not shameful because if you feel embarrassed by this the chances are that you will make whoever you are trying to communicate with feel awkward also, whereas they should probably be feeling proud of their language skill.

Generally most people respond well when you make an effort, even if you can’t speak their language fully. Responding to “Hola” (Spanish for hello) with “Hola”, saying “dankie” (Afrikaans for thanks), or whatever – little words. They’ll quickly realise you can’t speak their language fully, but appreciate the effort. Usually.

However, it’s those tourists that just walk in and expect to be understood in English, and will repeat the phrase in English louder – makes one shudder just writing it – that’s the brash other extreme that you want to avoid.

So, what I normally attempt is to initiate the conversation in their language – “Konnichiwa” (hello in Japanese) or whatever the local language is, and then ask “English?” or “hablas ingles?” (Spanish for do you speak English?) (ie if you know how to ask if they speak English in their language, go for that).

This prevents misunderstanding, prevents that awkward minute of embarrassment. You mention it’s harder when they initiate it – but for example, in Russia, where I speak maybe 10 words, I’d get a stream of words at me and I’d reply in broken Russian that I can’t really speak it. I learned how to say I’m from New Zealand, and speak English, and that’d usually convey it clearly and without much embarrassment at all.

I’d advise against just replying with “Hello” – maybe others would disagree, but to me it makes it seem like you’re overruling their language of choice. The “do you speak English?” asked immediately makes the situation clear, whether said in English or their own language.