The Ran in the gemara in Nedarim on 2a discusses what is considered a language and what is considered slang. It seems to come out that there is Hebrew which is an intrinsic language, and then there is everything else which is considered a language because it is an agreed upon communication between a people/nation; while Hebrew seems to not need the step of an agreement of people that it works as Hashem made it. We also see that when someone says a beracha in Hebrew, they successfully fulfill their obligation regardless of whether they know what they said or not (there are some things, however, that require your understanding and Saying it in Hebrew is not enough like the nullification of chometz before Pesach).
So, Hebrew needs to pronounced correctly if it is to be considered Hebrew and not a slang. But, this is where tradition/minhag makes things tricky. There are letters (such as gimel, daled, ayin, ches/chet, tes/tet) which are pronounced differently by different communities (vowels, too). Some communities have a continued tradition/mesora to pronounce these letters a certain way, but others don't.
It is important to pronounce words correctly, and especially Hashem's names. One who pronounces them correctly, and according to their tradition, is not creating a new language and it still remains Hebrew as opposed to a slang (though are those who would argue this point on some communities who may have pronunciations that seem to come more from dialectical differences as opposed to a tradition of pronunciation that was passed down, and thus potentially creating issues, according to some). So, by pronouncing things correctly, it's not a new language, rather it is called the correct pronunciation.
At the end of the day, you should pronounce things as written and be extremely careful to do so (just as someone who counts money carefully and with concentration - Mishna Berura in Hilchos Berachos). As far as questions regarding specific letters and vowels and what you should do, you should ask your local orthodox rabbi aka LOR.