We'd like to add a spellcheck dictionary in the Web Editor. It is not one of the languages listed. It is possible to add dictionaries to the system?

What format should they be in? How can we be sure that they are listed for selection?

The spellchecker uses Hunspell dictionaries which consist of an affix file (.aff file extension) and a dictionary file (.dic file extension). By default these files are located in this folder on the server:
C:\Program Files (x86)\Donnelley\MultiTrans 6.0\shared\HunspellDictionaries

Each language has its own subfolder whose name is its language code. For example, the folder for Canadian French would be called "fr_CA".

Here are a few sites where you can find Hunspell dictionaries for other languages:

• https://cgit.freedesktop.org/libreoffice/dictionaries/tree/
• https://github.com/wooorm/dictionaries
• https://github.com/ONLYOFFICE/dictionaries

When you add a new dictionary, you'll need to restart the services on the server before it will appear in the selection list. You can then make sure it's the default dictionary for that language by going to Settings > MultiTrans Settings > Spell check, then find the language and select the corresponding dictionary from the dropdown list: