Whenever a user switches the language during a survey, (s)he is presented the new language after that. So far, so good.
But.... in the results table of LimeSurvey only the startlanguage is stored, not the language that you change to. We ran into this when someone in the customer organisation sent the link with the dutch startlanguage in it to respondents in the UK. These people happily started filling in the survey, after changing this to English.
But they did not know that the resulting report would be based on... yes, the startlanguage.