"How does motivation influence language learning success, and what strategies can teachers employ to foster and sustain high levels of motivation among English language learners?"
Students who are more motivated apply themselves more to their lessons and over the long term develop higher proficiency. Teachers should help students envision future success that comes with higher proficiency and employ student-centered active learning.
Motivation is very important in language learning. As a matter of fact, many scholars and researchers suggest proposing materials that are interesting to them and that relate (somehow) to their interests. See, for example "SLA Research and Materials Development for Language Learning" by Tomlison 2017. See also https://files.eric.ed.gov/fulltext/EJ787917.pdf.
Motivation in language learning can be a "two-way street". It is not only that if students are motivated have a better performance, but students' motivation can be a motivation factor for the teacher himself/herself. In other words, s/he can learn from his/her students too!