Isn't it better to daven in your first language rather than davening in Hebrew and only knowing part of what you're sayig?
One is permitted to pray in any language that one understands. But why not just learn the translation and say it in the original?
[Effectively] everyone agree's all else being equal it is preferable to daven in loshon Hakodesh (see M.B. 101:13), the holy language. It also must be borne in mind that in other languages one must understand what is being said while in loshon hakodesh you have fulfilled your obligation even if you did not (M.B. 101:14)
The issue is that we are told that the malachei hashareis, ministering angels, do not understand Aramaic (See Shach Y.D. 335:3, Taz Y.D. 335:4, M.B. 101:15), which some (such as the Rif) understand to applies to all languages other than loshon hakodesh. While HaKadosh Baruch Hu certainly doesn't need to be informed by the angels of our prayers (He doesn't even need our prayers to inform Him of our needs or wants, or what we think we need or want), He created the world to be administered in a certain orderly fashion (See Derech Hashem 2, 6:1-3)
As such at times when our audience with Hashem is more direct, as it were, then it is easier to grant licence to pray in a foreign tongue. For example when one prays with a minyan an individual may pray in his native language (O.C. 101:4, MB. 101:17). Likewise when one is praying before a sick person it is permitted to pray in any language (Y.D. 335:5, M.B. 101:16) since the Divine presence resides there so to speak (See Taz 335:3, Shach 335:3, M.B. 101:17 based on Shabbos 12b).
An individual making personal petitions alone should pray in loshon haKodesh. (Shulchan Aruch, Orach Chaim 101:4)
The Biur Halachah on O.C. 101:4 explains that one who has yiras shamayim and wishes to pray in their native language specifically to improve their intent/concentration it is better to do so if they do not understand loshon hakodesh, otherwise loshon hakodesh has advantages which make it preferred.
I should add that in Hilchos Krias Shema the MB raised the issue of it being difficult to translate the text of the Shema in such a way that one has clearly fulfilled the Torah obligation. (I will try to add a more precise source).
On a practical note, after a couple days praying in English [for example] can easily become route if one isn't very careful. Being easier to understand doesn't mean it is easier to concentrate, indeed sometimes it is just the opposite.