Yes, it is. In fact all MS Office components use the same dictionaries and language functionalities, including e.g. search for synonyms, grammar, etc. It is just an indication that Outlook (MS Office) actually keeps the dictionaries quite separate, even when both are loaded.
Anyway, for me this is just an academic question. I hardly ever write emails in both languages and even then the “foreign” parts (be it German or English) are usually quotes and are inserted via _copy and paste. _
And I don’t use Outlook either, it just resides on my machine since you can’t uninstall individual MS Office components.
This whole thread is classified as idea , so it might be an idea to introduce some kind of AI that detects the language of a sentence or paragraph and chooses the dictionaries accordingly.
Thanks for the clarification Pefunk. There have been a few requests on this forum to include language tools other than a spell checker, so let us see what the developers have planned.
The last time I used MS Office it was possible to install only selected items. It has been so long that I guess the Redmond teams decided to change things. 
It is more than overdue that emClient at least allows to quickly change the spellchecker language (not with the keyboards – most people keep the native keyboard layout, even when writing in other languages!)
Better – and not really hard to implement – would be to automatically detect the used language.
It shows that the programmers from emClient do not use other languages. If they would need to switch the spellchecker language for every second e-mail (like I and others have to), this feature would be implemented the day before yesterday!
See my reply at https://forum.emclient.com/emclient/topics/button-to-quickly-change-spell-checker-language
I also beleive that auto language switching would be a really helpful feature. The software should support an auto language recognizion or at least allow 2 “active” languages for spell checking. Most companies have local and international mails to deal with, and having to manually switch the language for every second email is quite a hassle.
I really need the feature to switch between the German and English spelling dictionary on a regular basis.
At least it would be a fast - I know, not the best - solution to be able to switch between different spelling dictionaries using a dropdown in the tool menu and additional short cuts for this!
If auto detecting the active used language may be hard to implement, switching the language using a tool menu should definitely very easy!
Ok, forget about my comment! After searching more, I found the hint about using the right click menu! Button to quickly change spell checker language - #3 by Gary
The option to switch is in the More button in the toolbar:
Or you can add it to the regular toolbar through right-click > Add button:
And if you use eM Client on the Mac, the spelling is handled through the OS, so will automatically detect the language you are typing in, as it does in all Mac apps.