Should mantras always be sung in Tibetan/Pali/... or may I sing them in my native language? 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.

If your language is English, there are many pali chants in two languages in audio and here also as text.

(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.)