Here is the scenario. You want to set the primary language to UK English, but type multiple emails in French also, and don’t want to change dictionaries manually.
Copy the contents of C:\Program Files (x86)\eM Client\Dictionaries\fr-FR.dic, or a French word list from another application, to C:\Users\yourusername\AppData\Roaming\eM Client\en-GB.custdic. The format is important, so one word per line. It won’t work with a word list where the words are in a single paragraph separated by commas.
As long as your spell check language in eM Client is set to UK English, you will now also have the French dictionary in the word list.
The custom dictionary is named according to the language you have chosen to check spelling with. For German it will be de-DE.custdic. If the custom dictionary does not exist, start composing an email and add a misspelled word by right-click and choosing add to dictionary.