MS Word is anxious about your spelling, and for good reason. Many of us are rotten spellers as well as fumble-fingered. Word can help us if we approach this in a spirit of cooperation.
MS Word also worries about your grammar, but has less to contribute there since Word spent the time in Miss Pluperfect's English class flirting with PowerPoint and making fun of Excel. Excel refused to talk to anyone about decent English ever after and this explains why its help system is so incomprehensible (although better now than it used to be). But I digress.
Word is going to check your spelling unless you order it not to. Words that trouble it will show up underlined in red. Right-click and Word will offer alternative spellings in a range of relevance. (Control and click for Macs.)
If you hate the red underlines in general or because Word is flagging words that are spelled just fine, you have choices. 1) Learn to spell; 2) Turn spell check off; 3) Teach Word to spell. We will skip Choice 1 as hopeless and address the others.
The Options menu in MS Word allows you to turn off spell check (and grammar check) or to tailor them to your preferences. Take a look, but read about Choice 3 before you yield to irritation and go turning everything off. (Use the help system to find Options if its not obvious from the menus.)
Choice 3 is to tell Word that whatever it has underlined in red as deeply flawed is in fact just peachy the way it is. Right-click (Macs: Control-click) in the character's name or Polish town or dialect that you want Word to accept as fine. Select Ignore if you want a pass on only this instance, Ignore All for the whole document, and Add to Dictionary for every document you will ever create with this copy of MS Word.
Needless to say, you should be cautious with the Add to Dictionary option. "Mamah" or "likker" may not work in all your oeuvre, so perhaps better to stick with "Ignore All" rather than accepting it for everything forever. On the other hand, if you are writing a series and protagonist "Ivana" is flagged every time, go for it.
You can also go to Spelling & Grammar and use the dialog box to click through all the flagged words one after another instead of catching them on the fly.
Teach MS Word your idiosyncratic words, then take those red underlines seriously. Spelling matters if you want to come across as professional and competent. Find those words underlined in red and review the suggestions. You could be wrong, and Word could be right. This can save you embarrassment and possibly humiliation. Use all the tools you available to do the job right.
Picky, picky, picky.