Developmental dyslexia is a specific learning difference marked by persistent trouble with accurate and fluent word reading and spelling, despite adequate teaching, opportunity, and general ability. It is usually understood as language-based rather than visual, and it carries forward into second-language learning because reading an additional language draws on the same underlying machinery as reading the first.
The dominant explanation is the phonological-deficit hypothesis: dyslexia reflects a weakness in representing, storing, and retrieving the sound structure of language, which shows up most sharply in Phonological Awareness, the ability to notice and manipulate the phonemes inside spoken words. Because mapping letters to sounds is the foundation of Decoding, a phonological weakness makes that mapping slow and error-prone. Two refinements sit alongside this core account. The double-deficit hypothesis of Maryanne Wolf and Patricia Bowers (1999) proposes that rapid automatized naming, the speed of naming familiar letters, digits, or objects, is a second, partly independent source of reading difficulty, so that learners with both a phonological and a naming-speed deficit read worse than those with either alone. Limitations in Working Memory, particularly the phonological loop that holds verbal material briefly online, compound the picture by constraining how much sound-and-symbol information a learner can juggle while building a word.
In an additional language the same weaknesses resurface, often amplified. Decoding an unfamiliar orthography is effortful, phonological awareness in the new sound system develops slowly, and spelling lags. Because reading is laborious, Reading Fluency stays low, which starves comprehension and slows the incidental vocabulary growth that fluent reading normally feeds. Spoken production can be affected too, since holding and sequencing new phonological forms taxes the same memory resources. The transfer from L1 to L2 is captured by the Linguistic Coding Differences Hypothesis (LCDH) of Richard Sparks and Leonore Ganschow, which argues that foreign-language learning difficulties grow out of native-language difficulties: weaknesses in processing phonological, and to a lesser degree syntactic and semantic, information predict who struggles in the language classroom, locating the cause in language coding rather than in motivation or anxiety alone.
The standard pedagogical response is multisensory structured language (MSL) teaching, descended from the Orton-Gillingham approach to literacy. Instruction is explicit, systematic, and cumulative: sound-symbol relationships are taught directly and in a planned sequence, and each new element is rehearsed through sight, sound, and movement at once so that a weak channel is supported by stronger ones. Applied to a foreign language, this means teaching the phonology and spelling of the target language openly rather than expecting learners to absorb it, and revisiting it in small, frequent, well-structured steps.
The label itself is contested. Julian Elliott and Elena Grigorenko's The Dyslexia Debate (2014) argues that dyslexia, as commonly used, fails to mark out a scientifically distinct group: there is no clean line separating "dyslexic" readers from other poor readers, no single agreed definition, and no diagnostic test that reliably identifies a discrete condition. They contend that a diagnosis of dyslexia adds little beyond the observation that a child reads poorly, and may even divert resources toward those who can secure a label and away from equally struggling readers who cannot. The book also documents the abandonment of the older IQ-discrepancy model, the idea that dyslexia is reading attainment markedly below what intelligence predicts, which decades of research undermined, since discrepant and non-discrepant poor readers respond to the same instruction. The practical upshot, on this view, is to focus on identifiable reading difficulties and the support that helps them, rather than on defending the boundaries of a category. Defenders counter that dyslexia remains a useful clinical and legal construct and that the phonological account still explains a great deal; the debate is live rather than settled, which is itself the point teachers should carry.
Whatever stance one takes on the label, the instructional response converges. Teach the sound system and spelling of the target language explicitly and in sequence rather than incidentally. Build in multisensory rehearsal and frequent, low-stakes repetition so that fragile phonological representations get many chances to consolidate. Protect reading fluency with short, accessible, repeated texts before expecting comprehension at speed, and reduce the working-memory load by chunking and by not asking learners to decode and analyse simultaneously. Diagnosis is helpful where available but not a precondition for support: respond to the difficulty in front of you.