Shulchan Aruch, Orach Chayim 101:4:
When praying alone (i.e. not with a quorum of ten men), one should only pray in the Holy Tongue (i.e. Hebrew)… There are those that opine that even an one praying alone, when asking [G-d] for one’s needs, may ask in any language that one desires, except for the Aramaic language.
Mishnah Berurah, ad loc:
“one should only pray in the Holy Tongue” — because the Ministering Angels are not familiar with the Aramaic language, and so is the case for any language other than the Holy Tongue. However, [when praying] with the congregation, [the congregation] does not require an intermediary, for the Holy One Blessed is He himself accepts their prayers.
“even when praying alone… one may ask in any language… except for the Aramaic language” — [These opinions] maintain that the angels are familiar with all languages, but merely do not submit themselves to the Aramaic language because it is lowly in their eyes.
NOTE: These halachoth (laws) are based on the traditional idea that when not praying in a congregation of ten or more men, the Almighty only accepts one’s prayers through an angelic proxy that is created by the purity of intent of the individual that is praying. This is considered a much less efficacious form of prayer since the power of the angels to carry out this task may vary according to the intent of the one who is praying, as well as many other metaphysical factors and circumstances, while prayer with the congregation supersedes any of these external barriers to which angels may be vulnerable.