It depends very much on how deeply you wish to delve into the works of a particular philosopher. Many general concepts translate well, and you can learn a lot using only translated works in your native tongue. However, the devil is in the details. Many of the best philosophers pushed the boundaries of their language, and when one does so, it becomes remarkably hard to effectively capture the intent of the philosopher when undergoing translation.
I can give two examples. The first is an incredibly common case. Everyone who has interacted with Christianity in English knows "Thou shalt not kill." However, that is not necessarily the most ideal translation. The phrase in its native Hebrew is "לא ירצח". The word translated as "kill" is the verb "רצח". Most modern scholars believe that word is more effectively translated as "murder," not "kill," indicating an "unlawful killing," but with other meanings such as "to break, to dash into pieces." Needless to say, it gets complicated when one discusses "unlawful killings" in a document often referred to as "the law." For the majority of applications, the translation as "Thou shalt not kill" is sufficient. However, when discussing the ethical implications of warfare with respect to Biblical law, it is essential to know the difference.
The other example is a favorite of mine is found in Sun Tzu's Art of War (disclaimer: this was my own study as a layman who really doesn't know enough Chinese to do such interpretation. Of course, I think my child is perfect in every way, just like every parent!). This book is often recommended to business professionals to explore parallels between the business world and war. In the first chapter, as translated by Thomas Cleary, we find:
Therefore measure in terms of five things, use these assessments to make comparisons, and thus find out what the conditions are. The five things are the way, the weather, the terrain, the leadership, and discipline.
He then goes to talk about each of them. This is a fine translation, and captures a great deal of the intent. However, more meaning is found in the original phrasing of the second sentence (I provide a Pinyin pronunciation and gloss here, from (source). The source also has the original words, as written by Sun Tzu. Stack Exchange's text format doesn't allow Chinese characters, so I had to omit them here):
Yī yuē dào, èr yuē tiān, sān yuē dì, sì yuē jiāng, wǔ yuē fǎ.
[one] [say] [Way], [two] [say] [Heaven], [three] [say] [Earth], [four] [say] [General], [five] [say] [Method]
Just from the gloss, we can see minor differences. For one, "weather and terrain" have been glossed as "Heaven and Earth." This is where we can dig into the the cultural implications of Sun Tzu's actual words. "Dào" is a fundamental Chinese concept that you can spend a lifetime exploring and never fully understand it, but it can be described as the way everything flows (somewhat analogous to our concept of the universe). "Tiān" and "dì" are literally Heaven (or Sky) and Earth, so the gloss captures them well. In The Daoist cosmology, oft represented by three horizontal lines on top of each other, heaven is above (the top line), the earth is below (the bottom line), and man is in between (the center line). The next two characters are complicated. "Jiāng" is great fun, often thought of as a symbol for "meat" and "hand," referring to nourishment. It can be translated literally as General, in the military sense, but the word is also oft translated as "will," or even "future" ("will" being an essential human trait for putting food on the table every day). "Fǎ" is another complicated word with many translations. It has been translated as "law," "discipline," as well as many others (including the glosses' translation as "Method" with a capital M).
If one looks at it from this perspective, one has the way everything moves, heaven, earth, and the will and discipline of man in between. While the translation by Cleary may direct one to look at a set of things. The study into the original words suggests an alternate mindset Sun Tzu might have been inspiring: pay attention to Everything, categorized in the traditional Chinese manner of The Dao, Heaven, Earth, and ways of Men. Sun Tzu recommends a general never forget to perceive anything, never omit any detail presented before him.
I spent many hours on this sentence, trying to explore its meaning in its original language. I readily admit that I probably still got it wrong (shameless plea: any who speak Chinese, please correct any inadequacies in my work!). However, I find it an excellent example of what you can get from a translation versus what you can get with an in depth study in the native tongue.