Some application examples in summary:
If you write documents solely in your native language:
Simply set the desired default language on the Language tab in the dialog box of the ribbon command File | Options 
.
If you occasionally write entire documents in a foreign language:
In this case, it is not advisable to change the default language in the settings to this foreign language. Although this would lead to the desired result, it would only have a temporary effect. If you then set your original default language again and reopen the document, TextMaker checks the spelling and hyphenation in your original language again instead of in the foreign language.
It makes more sense to select the whole document, open the dialog box of the ribbon tab Home | group Character
and change the Language from Default to the desired foreign language. This enables TextMaker to know the document's language both now and in the future.
If you write documents with some foreign language citations:
Simply select the foreign language part, open the dialog box via the ribbon tab Home | Character and set the Language to the desired language.
Tip: You can do so even quicker via the Change language button 
on the ribbon tab Review | group Spelling.
You can change the language in a document as often as you like and even mix several languages.