How to determine only tweets of a certain language (en) in your search results?
searchresults1en <- searchresults1[which(searchresults1$lang=="en"),]
One can also directly query a language by using lang="en" like
lang="en"
en_f <- search_tweets( search_term, lang="en", n = 18000, include_rts = FALSE, retryonratelimit = TRUE )
en_f[en_f$lang == 'en',] is also possible
en_f[en_f$lang == 'en',]