Good question and one I've wondered about myself. Like you, I think it should be a data field on IMDb as well as searchable. For example, I'd like to be able to search for films released within a certain time period and then be able to filter that search for, say, just films dubbed into Spanish.

I attempted to get an answer to this by using IMDb's community forum. Previous questions didn't seem to answer what both you and I are seeking, so I posted a question on it. You can link to it here:

Is there any way to search for films by languages they have been dubbed into?

Who knows if it will receive a response. In the meantime, I have a few suggestions for you.

1) You can go to Amazon and search for a movie you're interested in. I'll use Blade Runner 2049 as an example. If you scroll down to "Product details," you will see that it lists "English, French, Spanish" after the field for "dubbed." Keep in mind that not every movie will have this field and I suspect that details on whether or not a movie has been dubbed into another language may not always be available. If this is the case, and you're pretty certain that the movie was likely dubbed into another language, check out other formats (or entries) for your film of interest to see if their pages don't have additional details related to dubbing. Take the movie Before Midnight, for example. This entry for it here, only lists English, but this entry here, also lists Italian (under language instead of dubbed). However, this box set here (if you didn't know, Before Midnight is part of a trilogy) lists Italian, English, French, and Spanish in that order (again under language and not dubbed). If you don't have a specific movie in mind, I recommend you use Google's advanced search. Type the following into the field for site or domain:

https://www.amazon.com

and then type

Dubbed

with or without a colon, followed by your language of interest (e.g., Spanish, French, German) and you should be quite pleasantly surprised with the results. I did this for Spanish and 42,700 results were returned and based off of a quick scan of the first page, it appears to definitely be in the ballpark of what might lead me to something of interest.

2) You can check out this site called DVDBeaver.com. Scroll down past its thumbnails of movies and you'll see a Google Custom Search bar. Type in the movie of interest and if its database has it, you'll be presented with a page that contains a lot of technical details about the movie that range from where to buy, type of production, and who produced it to runtime (down to the millisecond), video bitrate, and aspect ratio. For dubbing information, you want to look under the field for "Audio." I haven't read this anywhere, but I'm assuming the movie's original language is listed first and any additional languages it has been dubbed into follow. (Do not confuse "audio" with "subtitle.") For an example, take a look at its details for Logan's Run. I should warn you, however, that it might not be all that consistent in terms of its thoroughness. I tested this with another movie I know has been dubbed into Spanish — Before Midnight — and couldn't help but notice that it didn't list anything other than English for audio. However, the site is geared toward Blu-ray discs, so maybe that has something to do with it. Truth be told, the only reason I know Before Midnight was dubbed into Spanish is because I saw it aired in that language on television. Keep in mind that this same thing may happen with Amazon, too. (See my comments above.)

Additional notes

Pay attention to "region" information. I don't know if all sites give you more details about what these regional codes mean, but Amazon does a real good job of it. You can link directly to it here: "About DVD Region Specifications." It still doesn't tell you if the product in question has been dubbed, but it will tell you if it is a DVD or Blu-ray and which region it was designed to be compatible with.

Click on over to Box Office Mojo. This site doesn't provide some of the nitty, gritty detail that the others provide, but it does have a tab for "Foreign," which will show you all the different countries the movie was released in along with other interesting details to include the name of the distributor (although it looks as if you're more likely to see distributor name the newer the movie is). Also, be very careful with how you type in the movie name on this site. For example, "Bladerunner" (no space) won't return any results, but "Blade Runner" will.

Doing a search for something as simple as

movies that have been dubbed into

will produce some interesting results for you. When I did this for Spanish, one of the results that came back was this one from a Duolingo user:

How to Find American Movies with a Spanish Audio Track

I think you'll find some advice on that page worth exploring further even if Spanish isn't your language of interest.

As a last resort, you may want to try Wikipedia. It often contains really detailed information about a movie, even though its standard format isn't one that automatically includes dubbing information. It's worth checking out just the same. From time to time, you'll be treated to notes on dubbing that you might not find easily. For an example, take a look at the section on "Dubbing" for Suspiria. Just be careful with the "Plot" section if you haven't seen the film yet. Wikipedia generally does a very thorough job in this regard, and its level of detail could wind up ruining the movie for you and make you bypass an otherwise good movie you might have been inclined to see. It also has a category titled, "Dubbed films," but it focuses exclusively on films in languages such as Bengali, Hindi, Punjabi, et alia. If this is of interest to you, click on the link.

Consulting the work's page on the Wikipedia(s) of your desired dub language(s)

Google Translate goes a long way in helping that. Although unfortunately its translations of e.g. Asian languages can still be tricky to understand.

Translating web pages automatically with Google Translate

If you use the Google Chrome browser, it should automatically offer to translate foreign pages (but does not consistently do so for me).
If you want to test that: https://fr.wikipedia.org/wiki/Earthlings

If you use another browser, e.g. Mozilla Firefox, or Chrome doesn't offer the translation by itself:

Put this before the URL to translate:                               https://translate.google.com/translate?&u=

Example: https://translate.google.com/translate?&u=https://fr.wikipedia.org/wiki/Earthlings

Unfortunately, the Wikipedia approach is not very reliable. While the French Earthlings page is maximally helpful in that, right in the introduction, it names all official dubs (even with their respective narrators) and points out that inofficial ones have been published on the internet, in the Spanish one there's only an easily overlooked link at the bottom of the page ("doblado" translated to "folded" by Google Translate).

Googling specific dub languages

Example: https://www.google.com/search?q="english"+dubbed+the+intouchables

(Putting single search terms within " double quotes  makes sure they are actually present in the results. (You could also apply that to the word dubbed.))

_{(I know this approach is super obvious, but I think it should be here for the sake of completeness and so it can be ranked against other approaches through votes.)}