I have just learnt that 'hanasemasu' means 'to speak' and that 'ga hanasemasu' means roughly the same. On Google Translate, 'hanasemasu' comes up as 'able to speak' and 'ga hanasemasu' comes up as 'I can speak', but as far as I know, there is no conjugation in Japanese, so if you want to specify 'I can speak', you would add 'watashi wa' at the start. I have also seen it written as 'wa hanasemasu'. So my question is: what does the 'ga/wa' mean?
hanasu / hanashimasu means "to speak"
hanaseru / hanasemasu means "to be able to speak"
"X-san ha (pronounced "wa") Y-go ga hanaseru / hanasemasu" means "X-san can speak Y language".
As other people have mentioned "watashi ha" does not have to be inserted when talking about yourself, because in Japanese, in a phrase with "no subject", the subject is understood by context. If there is NO context, the assumption becomes that you must be speaking about yourself, so that
"nihongo ga hanasemasu" becomes "I can speak Japanese."
in such a case, simply speaking, the "ga" connects the topic and the verb.
The 'ga' in your example there is hanging there by itself doing nothing. You need a word before it. For 'hanasemasu' (by the way which means 'to be able to speak' not 'to speak'), the 'ga' is generally the marker for the language you can speak. So 'nihongo ga hanasemasu'. You can add 'watashi wa' at the start to indicate that it's you who can speak Japanese, but it can generally be inferred.