Greetings,
A Mantra works at two levels 1) Psychological 2)Astral
1) Psychological level: The mantra will induce the meaning it is conveying into your mind and psyche e.g.: A mantra on love will develop intense feelings of universal love within you, A mantra on health will induce vigor and positive thinking.
To achieve these results language is immaterial, but what matters is the attitude, devotion and focus with which you chant those mantras.
2) Astral Level: Yoga describes a human body as consisting of several energy centers (called as Chakras). These energy centers govern your whole being. Each Energy center has a corresponding sound, color and geometrical representation to it. Sanskrit mantras are designed to activate and stimulate these Energy centers precisely. The repeated sound vibrations produced during a chant affect these energy centers in a positive and impacvtful manner. Henec for this purpose Sanskrit is most suitable. Hence even a lack of sincere attitude during chanting will give good results due to the physical impact of sound vibrations on those energy centers.
NOTE: Sanskrit mantras are not dveleoped by anyone. It was discovered by sages/saints during their high/intense stages of divine communion. It was revelaed to them and has been practiced for thousands of years with astonishing success.
My personal preference are Sanskrit mantras. Once you know the meaning then you can relate to every pronunciation. Though you can continue in English as long as your devotion and attitude are correct.
Best of Luck
