First of all, secular Jews do not in general pray in any language, so that is irrelevant.
Second, there is a major difference that must be made between Hebrew as a secular/spoken language versus Loshon HaKodesh (LK), which is the language of Torah, kedusha and prayer.
Religious Jews who have some understanding of LK pray in LK as that is the language prayer was written in. If a person really does not understand LK (such as a baal teshuva) he can pray in his own language until he hopefully gets enough proficiency to be able to pray in LK. The difference for an Israeli baal teshuva is that he may understand enough LK because his mother tongue is Hebrew, but they are not the same.
The people from the Haskala wanted to remove the kedusha from LK and make Hebrew into a spoken language again.
If G-d revealed Himself in LK to the Jewish People, what makes you think that people in this world can replace that language with something else and keep it holy? Their only purpose was to remove the holiness (kedusha) and make it a common, everyday language.