It is very proper for all to learn the meaning of the words of Shemoneh Esray, especially Avos and Modim, as there is an obligation to know what you are saying. (MB 2)
If you cannot have kavana for the meaning of all of the berachos, at least have kavana for Avos and Modim and at the ending of each beracha. (ShA1 – MB 1,3)
If you figure that you presently cannot manage to muster your kavana for even Avos, do not daaven until you can. (MB 3)
If, at the end of Avos you remembered that you didn’t have kavana, as long as you did not yet say “Boruch atta Hashem…” go back to “…Elokay Avraham…” and say it with kavana. If you remembered after that, just continue. (Rama 1, MB 4)
You must enunciate the words of Shemoneh Esray and whisper them loud enough for you to hear yourself, but soft enough that the one standing next to you cannot hear. (ShA 2 – MB 6)
If you can’t concentrate by saying the words so softly, you can adjust your volume to enable kavana. Just to be sure not to disturb others. (MB 6)
Also, during Pesukay D’zimra it is best not to raise your voice too high. (MB 7)
When you are not daavening with the tzibbur, you may raise your voice as loud as you need to help your kavana or to teach the members of your household how to daaven. (ShA 2-MB 9)
With the exception of Aramaic, you can daaven in any language that you want to as long as you understand it. However it is a mitzva min hamuvchar to daaven specifically in Lashon HaKodesh and then you are yotzay even if you do not understand it. Besides, Chazal chose the words of Shemoneh Esray very carefully – they are packed with segulos and hidden, lofty secrets. Even if you don’t know their intent, the holy words themselves rise to the proper place. (ShA 4 – MB 13,14 – BeH “yachol”)
When daavening with a tzibbur, you do not need malachay hasharays to intervene, therefore we find the tefillos such as Y’kum Purkan and Breech Sh’may said in Aramaic. When daavening alone, omit such tefillos. (MB 15,19)