I am very interested in adding mantra singing to my practices. But sometimes it feels kind of weird singing in a language I do not speak. Is it possible to sing my own prayers or will the effect change, once the sounds are different?
Since it seems that Mr Marcos Valle wishes and beg form somebody, its good that he/she/they understand you wishes, speak this languages. As for Mr. Marcos Valle it would be good to know what he wishes for, given that they will fulfill simply what he requests. :-) not easy, especially the fist one.
As for chants (repeating what other said, or what one likes to say), they are not well put, if sung. It could district not only your mind but also the listener and they could argue about it or fall even in love with it. (for some ideas about it, see "Teaching Dhamma" in Respect-BMC2)
Its quite ok and good, to use the own language, especially if one does not know the meaning. And you are explicit allowed to do so even with the Buddhas teachings, by the Buddha, to do so. (see also BMC2 which give a good reasoning) So no worry, but keep them correct in their meaning and be clear about it. Many Monks today would chant in two languages, so that the listener understands. There are no such as simply chants, which are actually not teachings. So one does not have benefit form simply hearing a pleasant sound. But to get and share the meaning, or reflect on it while doing.
Chanting generally is primarily done to do not forget and preserve the teachings for later generations and others to listen.
(Note: this answer has not been given with the agreement to be means of trade or the purpose of/for trade and/or keep people trapped and bound. How you handle it lies in your sphere, but does not excuse the deed here either.)
The origin of mantras starts with an old Indic idea that Sanskrit was the original language and had the power to directly create reality. The closest thing we have to this in modern linguistics are performatives, e.g. "I now pronounce you man and wife" in a sense, makes it so. If you accept this reasoning, you should say mantras in Sanskrit. As far as I can tell, this is the means by which mantras are expected to work in Buddhism as well.
Mantrayana spread to many non Sanskrit speaking cultures, China, Tibet, Japan, etc, where the mantras were transliterated into the local language, but not translated. In the case of Japanese, it went through two phases, Sanskrit to Chinese, then Chinese to Japanese. Now Americans chant Japanese mantras in English, mostly just losing the Japanese accent and the long and short vowels.
In Shingon (Japanese Vajrayana), the founder Kūkai said Matras were the mantras that meant something while the Dhirani were the ones that did not. Om ah hum, doesn't translate as clearly as say, Namu Myoho Renge Kyo (Homage to the Wonderful Dharma Lotus Sutra). In the case of Dhirani, it really doesn't matter what language because they are not translatable to any language, certainly not English.
In Nichiren Buddhism, you chant the Japanese pronunciation of a the Chinese translation of the Lotus Sutra. You are not expected to understand it while chanting it. In that system, you are appealing to the mystical principle, the dharma, the way things are to work in your favor or to enable you get the things you need to ultimately achieve Buddhist goals. That's a long winded way to say, in Nichiren Buddhism, you unashamedly chant to get a promotion, your next meal or a new car.
In my personal practice, I treat mantrayana as "noisy meditation." I'm more keen to see if I can use repeating a phrase to achieve the same ends as concentrating on the breath. Since that is my goal, I tend to do mantras in what ever language is easiest to pronounce, typically Japanese. Once a phrase has been repeated dozens of times, you start to stop paying attention to what it might mean anyhow.
In other traditions, though, like Amidaism, you are supposed to recite the Amitabha's mantra while focusing on your aspirations for attaining the Pure Land, or your gratitude, depending on who you are reading.
It seems to me, perhaps mistakenly according to the "masters," the meaning of the words or the spirit in which they are chanted would be of higher importance than the language used to speak them. Of course, then the questions would be: What is lost in translation and what do you mean by spirit? Perhaps a translation from the Sanskrit directly to English would be the best path, if one is willing to take someone else's translation as accurate with no other evidence than their professed expertise. Or...maybe just forget all about that mantra chanting! Each path has it's own footprints so maybe your path or practice should be the one you work out for yourself, rather than accepting the received "wisdom" of others. Who's rulebook do you adhere to? Rules...pfft!...rules, rules, rules. Chant that instead, maybe.