This paper presents the application of rule-based methods for Grammatical Error Correction (GEC) across multiple low-resource languages. We describe new functionality using the Constraint Grammar (CG) formalism, designed for detecting and correcting different types of complex grammatical errors in a range of morphologically complex languages. These errors require transformations such as reordering, word additions/deletions, and alternative choices for multiword suggestions. New perspectives are gained from end-to-end-testing – this work aims to clarify the relationship between the command-line interface used by developers and the user interfaces of our grammar checker plug-in for common word processors. We discuss challenges and solutions in correcting complex errors, with examples from languages like Lule Sámi, Irish, and Greenlandic, enabling linguists to adapt these methods in order to provide accurate and context-aware proofing tools for their own languages in mainstream word processors like Microsoft Word, Google Docs or LibreOffice.