Learning any language helps learning another language after faster. This propaedeutic value is found in all language, but is more noticeable for some languages and combinations. For exemple, for elementary school, two years of Esperanto + four years of English is better than six years of English alone for English proficiency. I wonder what makes a language propaedeutic. Maybe with this knowledge it is possible to make an optimal propaedeutic course depending of the goals of the language learner.
Having studied several languages including some Esperanto myself: The most obvious factor is if two languages are similar, either in vocabulary or grammar, already knowing one will make the other easier. Beyond this, when learning a second language, one can familiarize themselves with which techniques work and don't work, and thus waste less time. In addition, one can have more confidence in the third language from the start due to previous successes in language learning. Now, if one's goal is to learn English, learning a language like German first will just delay the final goal, as German is just as difficult as English. However, learning a very simple and regular language like Esperanto, where a dedicated learner can become conversationally proficient in just a few months, will allow one to gain the benefits of having already learned a language, but without having diverted too much time and energy away from their original goal.
Of course, this is not only my opinion. Here is an excerpt from a linguistics book by Dr. Timothy Reagan discussing why Esperanto makes a great propaedeutic language:
Thus, Esperanto has a number of propaedeutic advantages, among which are:
Esperanto is easier, and faster to learn than are national/ethnic languages. This claim is most commonly manifested in comparisons of the learning time involved in acquiring competence in Esperanto with that needed for a national/ethnic language.
Esperanto can be taught more efficiently and explicitly than can national/ethnic languages. The basic idea here is that while national/ethnic languages are generally best taught inductively, this is not necessarily the case with Esperanto, and, further, that direct, explicit instruction will make both teaching and learning more efficient. The fundamental idea here is that because Esperanto strives to avoid the irregularities, inconsistencies, and complexities of national/ethnic languages (while at the same time preserving the fullest possible degree of linguistic expression), several assumptions are commonly made about its learnability. Specifically, it is often argued by advocates of Esperanto that (a) Esperanto is more learnable (i.e., that any given level of proficiency can be achieved in Esperanto with less effort and in shorter time than with any national/ethnic language) and (b) that its superiority in terms of learnability is much greater than any inherent biases in its vocabulary or structure that would favor speakers of some languages (e.g., speakers of Indo-European languages) over others.
The learning of Esperanto has propaedeutic effects with respect to the learning of other languages, and perhaps even other subject matter. In other words, the learning of Esperanto will reduce the time needed for the learning of other language (and perhaps other subject matter, such as geography, mathematics, etc.). As an investment, in short, learning Esperanto may have significant benefits unrelated to the acquisition of the international language itself.
The learning of Esperanto has a positive effect on students' self-confidence. As a result of successful learning of Esperanto, students will feel more competent and empowered, and this will result in a higher level of selfconfidence (which, in turn, can result in improved learning in other areas).
Learning Esperanto is especially appropriate for students with special needs (including both gifted/talented students and those with learning disabilities). Although different arguments are used, claims have been made that students at both extremes of the population may have greater interest in and success with Esperanto than with other types of subject matter (Quick, 1989; Wood, 1975).
In short, the regular structure of Esperanto and its ease of learnability make it an ideal choice for developing second language learning skills for students (Maxwell, 1988). Although all language learning is difficult, learning a language like Esperanto, especially as a first foreign language, can promote positive experiences with and attitudes toward additional language learning.
Reagan, T. (2010). CHAPTER FOUR: Critical Pedagogy in the Foreign Language Education Context: Teaching Esperanto as a Subversive Activity. Counterpoints, 376, 47-66. Retrieved from http://www.jstor.org/stable/42980732