It won't suffer at all and research shows that in fact, it might be improved. The September 2010 research report commissioned by Bòrd na Gaidhlig from Edinburgh University* concluded that their results were consistent with previous findings that Gaelic-medium pupils’attainment in English had overtaken their attainment in Gaelic by the end of primary: 83% of Gaelic-medium pupils passed Level D Gaelic reading, compared with 93% achieving Level D English reading; the difference was similar for writing.
*GME in Scotland:Choice and Attainment in Primary and Early Secondary School www.gaidhlig.org.uk
Children in Gaelic-medium education are taught entirely through the medium of Gaelic in the Early Years, to ensure that they develop a good grounding in the language. English language lessons are introduced at Middle Stages and by primary seven children are exposed to an equal balance of Gaelic and English.
The Edinburgh University research report referred to above cited that pupils in Gaelic-medium education, who are not exposed to English in the classroom until at least Primary 3, catch up with and overtake English-medium pupils in their command of English