If I understand the problem correctly, when you add a user-defined command, like in my case
\remark, TexStudio somehow add this new command to a known-commands list. (Thereby it is able to manage auto-completion on user-defined commands.) But when it does so, it cannot guess the type of parameters that your command is supposed to take. I don't know what happens precisely here, but for sure the default comportment is not to consider that your new command takes text as parameter.
Therefore, if you want, as I did, to add a new command that will have some text as parameter you have to create a
.cwl file in order to explicitely specify the type of your command's parameter. In my case I added a file
~/.configure/texstudio/. In this file for now I only have a single line:
After doing this, once you restart TexStudio, you can go to
Options -> Configure TexStudio -> Completion and check
mycommands.cwl. Now you command is recognized as having text for both parameters and TexStudio will thus spellcheck its content.
For more information on .cwl file format you can refer to TexStudio's manual.