Previously these files were created by Edge, a Chrome based browser but adding a language now doesn't create these files.
So unfortunately I don't know a good way of finding these files.


Chose the file(s) you need from:
https://1drv.ms/f/s!Ag30khKnxvoqiKc0Ad9swnBBfidkNA

I'm currently trying to find a repository on the Internet, but without luck so far so please leave a comment below if you have dictionary files for other languages.

Version 1.0
This section won't include any images because it doesn't make sense as there are so many ways of acquiring these files.

To get the .bdic file you need to have an application which uses these files in their spell check, such as Chrome based web browsers, there might be other applications which uses these files too so feel free to chose any of your liking.

When you have installed or already have such an application, go in to settings and add another language for spell check.
This will in turn create the .bdic-file if it wasn't there to begin with.

Other operating systems

I wouldn't know, perhaps the same applies for those?

Open an explorer window(Windows+E) and search your computer by expanding "This computer" which is the top node of the explorer tree and select C: then type "*.bdic" without the quotation marks in the search box.
If nothing is found you can search your remaining hard drives provided you have more than one using the same method.
This may take a while depending on the size of your hard drive and other things as well.
When completed you should have a list of locations where Windows found the files.
NOTE: One of these files will be the Steam spell check file for your current language, the path contains "...CEF". Ignore it for now.

Select the files for your languages you want Steam to spell check and copy them(Ctrl+C)
Now browse to "%localappdata%CEFUser DataDictionaries" (If you select and copy* the text between the quotation marks and paste them into the Windows explorer path-box you automatically end up in the correct folder)
Here you see the file mentioned in the NOTE above

Paste(CTRL+V) the files in this location.

* Which will be counter productive seeing it'll clear the clip-board. But I'm sure you manage to resolve this on your own.

Now it's time to modify the .json file which controls the spell check.
Still in "%localappdata%CEFUser DataDictionaries"

Write the letters between the first "-" in the file names for the languages you want Steam to spell check. Preferably in a Notepad window which you open separately.
Example:
en-GB-8-0.bdic, write "en-GB"
sv-SV-3-0.bdic, write "sv-SV"

Now open the UserPrefs.json-file located in "%localappdata%Steamhtmlcache" with Notepad or any other non-rich text editor.
Find the section where it say "dictionaries"
NOTE: There might already be text within [".."], just ignore that for now

Erase everything withing the square brackets and type the language codes you wrote down earlier
"dictionaries":["l1-L1","l2-L2","l3-L3","..."]
For Swedish and English like I have
"dictionaries":["en-GB","sv-SE"]

The complete .json.part
"spellcheck":{"dictionaries":["en-GB","sv-SE"],"dictionary":""}

Save the UserPrefs.json-file

Re-start Steam if you had it running, or start Steam if you hadn't.

Now when you type in chat-boxes or community text-boxes wrongly spelt words will have the red waves under them.

If this for some reason wouldn't work I suspect the text in the UserPrefs.json is wrong.
Please make sure the language codes are correct and that the format is exactly as the example.
Otherwise, please ask in the comment section below.

This still won't make you able to replace text by right clicking on the marked word as we're used to in regular cases. Perhaps Valve fixes this some time in a near or distant future.
Valve has fixed this now both for forum comments and chat.
If you right click a misspelt word Steam chat/comment will now suggest a word for you. Some words such as "misspelt", ironically enough doesn't exist in the library.
The suggested word appears in two different places under the menu so it's not consistent yet.
But it's something!

Either if you thought this guide was helpful or not or has to improve in some areas, please let me know in the comment section below.