Set the proofing language
Select all (Ctrl+A) → Review → Language → set the real language. Then check that the style Normal is not locked to another language. Mixed documents (English abstract, Arabic body): select each run. A single “detect language automatically” pass will mis-tag names and code.
Install the pack
If the language is missing, Settings → language / download proofing. UI language and proofing language are different switches. English menus with German squiggles is a valid setup for a bilingual office.
Ignore vs dictionary
Add your surname and your product name to the custom dictionary. Do not Add All on a scanned OCR dump — you will teach the dictionary garbage. “No red lines” on a wall of typos means proofing is off, language is “do not check,” or the text is an image. OCR first: PDF.
RTL
Arabic and Hebrew need a font that contains the script and a paragraph direction. If punctuation flies to the wrong side, you applied LTR to an RTL paragraph. Fix the paragraph, not each mark.
Mixed-language paragraphs
A paper with English abstract and Arabic chapters needs two proofing languages, not one autodetection pass. Select the abstract → English. Select the body → Arabic. Names of English institutions inside Arabic text will always look “wrong” to the Arabic checker — Ignore Once, do not Add to dictionary unless you want them ignored in every file.
ALL CAPS
Spellcheck is weaker on all-caps headings. That is not a reason to write the thesis in caps. Title Case or Sentence case. Small caps as a style if the department demands a look.