A learner corpus, also called interlanguage or L2 corpus, is a computerised textual database of language as produced by second language (L2) learners. Such a database is a powerful resource in research of second language acquisition. It can be used to optimise the L2 learning process, to assist authors of textbooks and dictionaries, and to tailor them to learners with a particular native language.
Czesl, a learner corpus tool for Czech, has been written in Java on top of the NetBeans Platform. The screenshot below shows the annotation of a sample sentence as displayed by the tool.
More details (PDF): https://www.aclweb.org/anthology/W/W10/W10-1802.pdf