A spell-checker is only as good as the dictionary behind it, and that is never more obvious than when you write in more than one language. Type a French sentence into an English checker and it flags almost every word; leave the accents off and even a French checker may lead you astray. Multilingual writing rewards a little setup and a clear understanding of what the tool is doing.
Why a generic checker fails on other languages
Remember that a checker simply compares each word against the word list it searches. An English dictionary does not contain bonjour, gracias, or München, so it marks them all as errors even though they are spelled perfectly. Worse, it may “helpfully” suggest an English word that looks similar, tempting you to accept a change that turns correct foreign text into nonsense. The fix is not a better checker but the right one: set your program's language to the language you are actually writing.
Accents and special characters matter
In many languages an accent is not decoration — it changes the word. In French, a (has) and à (to) are different words; in Spanish, the tilde in año distinguishes it from a very different word without it. Dropping accents is a genuine spelling error, not a shortcut, and a properly configured checker for that language will catch missing ones. Modern software handles these characters through the Unicode standard, which gives every character in every language a consistent code, so accented letters display and check correctly across programs and devices — provided your keyboard or input method can produce them in the first place.
Switching languages within a document
Many word processors let you mark different passages as different languages, so a quotation in German inside an English article is checked against the German dictionary while the rest stays English. This is the cleanest way to handle bilingual documents: the checker applies the right word list to each part instead of flagging everything foreign as wrong. If your program does not support per-passage languages, the practical alternative is to check each language's text in a separate pass with the dictionary set accordingly.
Specialized dictionaries: medical, legal and technical
Some vocabularies defeat even a correctly chosen language dictionary because the words are simply not in general use. Medicine, law, engineering, and the sciences each have thousands of precise terms that a standard checker flags on every line. Specialized dictionaries and word lists exist for exactly these fields, and adding one — or building your own list of the terms you use most — turns a constant stream of false alarms into a checker that actually helps. In high-stakes fields the payoff is real: a misspelled drug name or legal term is not a cosmetic error, so a dictionary that knows the vocabulary is worth the setup.
Autocorrect across languages: a special hazard
Automatic correction is helpful within one language and treacherous across two. If your device is set to English and you type quickly in another language, autocorrect may “fix” correct foreign words into English ones without asking — turning a fluent sentence into gibberish that is, ironically, perfectly spelled. The problem is worst in short messages and chat, where you are typing fast and not rereading. When you switch languages, switch your keyboard or input language too, and give anything important a final read before it is sent. A message that autocorrect has quietly translated word by word is one of the few writing errors that can be both invisible to you and glaring to your reader.
When in doubt, confirm the word
No checker replaces knowing the language, and for anything important a second source is wise. A reputable bilingual dictionary confirms both the spelling and the meaning, which matters because a word can be spelled correctly and still be the wrong word — the same trap that exists in English, multiplied across languages. The same principles of using a dictionary well apply in any tongue, and the discipline of English spelling variations — choose a standard, set your tools, stay consistent — carries over directly to writing in any language.