Thanks for this detailed info. This really helps. 
(I partly got brought on board to fix up + maintain Collabora’s documentation. In the previous 2 years, I wrote nearly 1000 LibreOffice step-by-step instructions + tutorials!)
This is perfect—because I’m pretty new to the whole Collabora Online too—so we’re both looking at this as users with “fresh eyes”. 
So, if I’m summarizing you correctly…
You were also having trouble finding instructions for:
- Docker installation
- “Change Document’s Language” instructions
- Spelling / Dictionary / Thesaurus
- Especially for:
- All “New Documents”.
- Defaulting to “German” instead of English.
And, it would be nice to have an easier way to:
- Change document’s default language.
- Especially with the Tabbed UI!
Thanks for all this detailed info though, it’ll really help make Collabora better. 
How did you find out about the collaboraoffice-dict-en
package?
I didn’t find any instructions (sadly).
Hmmm, I did a quick search through the Collabora Online SDK.
It took me quite a while to find, but I found it mentioned in:
Spelling dictionaries and thesauri
Collabora Online can use internal spelling dictionaries and thesauri (collaboraoffice*-dict-*
packages). Collabora Online can use system spelling dictionaries and thesauri, too, that are located in /usr/share/hunspell
and /usr/share/mythes
directories.
I’ll mark that down as an area that needs updating. 
Very hard to find in search. Even when I was using keywords like:
language
collaboraoffice-dict
(And no normal human uses the word “thesauri”!)
Side Note: Another trick you can do is type this into your favorite search engine:
spelling site:https://sdk.collaboraonline.com/
That will help search through ONLY the SDK instructions as well.
Further, there are no clear instructions to setting a default language when creating a new blank document in NextCloud (tried everything so far to no avail).
Lastly, just setting a document’s language isn’t documented anywhere for the CODE. […]
Hmmm… in Collabora Online 23.05, looks like language changes are different in the “Tabbed View” compared to the “Compact View”. See the tutorials below.
As a user, there’s also the language in the bottom status bar:
Last year, for LibreOffice, I wrote a tutorial describing the 3 different ways to change language:
- Menu
- Status Bar
- Styles
See the post here:
Collabora Online’s are similar, but different…
AND, we have a 4th extra one on top… because:
- The server (ADMINS/Docker/Nextcloud) can decide what the default document language is (?)
which may cause an extra layer on confusion. Like let’s say your case:
- A “German” admin
- An English user
I suspect you’d want a newly created document to be based off the user’s language.
Changing Document’s Language (Collabora Online 23.05)
The menu options currently exist in Collabora Online, but you have to use Compact View instead!
1. Go to:
2. This will get you the:
menu with all 3 options:
- For Selection
- For Paragraph
- For All Text
- This will change the entire document’s language!
After, to get back to Tabbed UI, press:
Everything I’ve seen is about LibreOffice and the most obvious toolbar icon for setting the language for the entire document, a paragraph, or selection simply doesn’t apply to the default of the document (despite being labeled “entire document”); that feature applies strictly to the existing content of the document and not the document itself (confusing).
Hmmm, so if you:
- Set 100% of the document’s text to one language
… then:
- the document’s language doesn’t change too?
Definitely sounds like an issue we could report (to the Github)!
I swear that’s how it used to work in a slightly older version of LibreOffice.
(But I just retested LO 7.5.4, and the Status Bar method doesn’t work that way. Wonder if something changed within the past year?)