Hey @TurabG, thanks for the questions + welcome to the forum. 
Which language are you using?
I explained how to change the UI to “Portuguese (Brazilian)” in this post:
At the end of your URL, you have to stick:
&lang=pt-BR
where pt-BR is whatever UI language you are trying to use.
On Default UI… see answer earlier in that same thread too:
Did you see the Collabora Online SDK:
Method #1: Docker Environment Variable
At startup, you can pass your docker an environment variable:
dictionaries
Method #2: coolwsd.xml File
The list of languages + how to expand them is also described in the Collabora Online SDK:
If you look inside the file:
coolwsd.xml
you should see a big list of:
<allowed_languages desc="List of supported languages of Writing Aids (spell checker, grammar checker, thesaurus, hyphenation) on this instance. Allowing too many has negative effect on startup performance." default="de_DE en_GB en_US es_ES fr_FR it nl pt_BR pt_PT ru">de_DE en_GB en_US es_ES fr_FR it nl pt_BR pt_PT ru</allowed_languages>
You can adjust those languages as needed.