If a document uses multiple languages, or has been translated, spellchecking may function incorrectly.
The language used for spellchecking is determined by "Paragraph Styles" that are defined by the artworkers. It is important that the defined styles are adjusted accordingly.
Please contact your agency who will be able to help you solve the issue. They can use the information below.
For InDesign users:
Double-click on the style in the "Paragraph Styles" palette. Check that, under the "Advanced Character Formats" option, the "Language" is set appropriately. See image below: